http: cork/uncork before flushing pipelined res

Make sure that the pipelined response data will be written as less TCP packets as possible.

Benchmark results (3 runs):

http/chunked.js num=1 size=1 c=100: ./node-after: 2358.9 ./node-before: 2374.1 ................................... -0.64%                                                                           [87/709]
http/chunked.js num=1 size=64 c=100: ./node-after: 2372.7 ./node-before: 2373.2 .................................. -0.02%
http/chunked.js num=1 size=256 c=100: ./node-after: 2374.2 ./node-before: 2367.9 .................................. 0.27%
http/chunked.js num=4 size=1 c=100: ./node-after: 2363.1 ./node-before: 2356 ...................................... 0.30%
http/chunked.js num=4 size=64 c=100: ./node-after: 2370.7 ./node-before: 2358.6 ................................... 0.51%
http/chunked.js num=4 size=256 c=100: ./node-after: 2364.8 ./node-before: 2355.7 .................................. 0.39%
http/chunked.js num=8 size=1 c=100: ./node-after: 2231.5 ./node-before: 2332.9 ................................... -4.34%
http/chunked.js num=8 size=64 c=100: ./node-after: 2253.1 ./node-before: 2357.3 .................................. -4.42%
http/chunked.js num=8 size=256 c=100: ./node-after: 2346.7 ./node-before: 2300.2 .................................. 2.02%
http/chunked.js num=16 size=1 c=100: ./node-after: 2248.2 ./node-before: 2076.3 ................................... 8.28%
http/chunked.js num=16 size=64 c=100: ./node-after: 2223.2 ./node-before: 2052.4 .................................. 8.32%
http/chunked.js num=16 size=256 c=100: ./node-after: 1928.6 ./node-before: 2133 .................................. -9.58%
http/client-request-body.js dur=5 type=asc bytes=32 method=write: ./node-after: 3296.9 ./node-before: 49.987 ... 6495.58%
http/client-request-body.js dur=5 type=asc bytes=32 method=end: ./node-after: 3308.6 ./node-before: 49.598 ..... 6570.96%
http/client-request-body.js dur=5 type=asc bytes=256 method=write: ./node-after: 3462.5 ./node-before: 49.595 .. 6881.48%
http/client-request-body.js dur=5 type=asc bytes=256 method=end: ./node-after: 3520 ./node-before: 49.599 ...... 6997.02%
http/client-request-body.js dur=5 type=asc bytes=1024 method=write: ./node-after: 3384.3 ./node-before: 49.59 .. 6724.59%
http/client-request-body.js dur=5 type=asc bytes=1024 method=end: ./node-after: 3409.9 ./node-before: 49.594 ... 6775.71%
http/client-request-body.js dur=5 type=utf bytes=32 method=write: ./node-after: 3373.4 ./node-before: 49.597 ... 6701.50%
http/client-request-body.js dur=5 type=utf bytes=32 method=end: ./node-after: 3319.9 ./node-before: 49.991 ..... 6540.99%
http/client-request-body.js dur=5 type=utf bytes=256 method=write: ./node-after: 3606.5 ./node-before: 49.994 .. 7113.99%
http/client-request-body.js dur=5 type=utf bytes=256 method=end: ./node-after: 3211 ./node-before: 49.592 ...... 6374.84%
http/client-request-body.js dur=5 type=utf bytes=1024 method=write: ./node-after: 2963.8 ./node-before: 49.585 . 5877.28%
http/client-request-body.js dur=5 type=utf bytes=1024 method=end: ./node-after: 3298.2 ./node-before: 49.986 ... 6498.27%
http/client-request-body.js dur=5 type=buf bytes=32 method=write: ./node-after: 3454.9 ./node-before: 49.99 .... 6811.27%
http/client-request-body.js dur=5 type=buf bytes=32 method=end: ./node-after: 3512.3 ./node-before: 49.599 ..... 6981.38%
http/client-request-body.js dur=5 type=buf bytes=256 method=write: ./node-after: 3459.6 ./node-before: 49.595 .. 6875.69%
http/client-request-body.js dur=5 type=buf bytes=256 method=end: ./node-after: 3642.6 ./node-before: 49.99 ..... 7186.51%
http/client-request-body.js dur=5 type=buf bytes=1024 method=write: ./node-after: 3522.2 ./node-before: 49.989 . 6945.95%
http/client-request-body.js dur=5 type=buf bytes=1024 method=end: ./node-after: 3551 ./node-before: 49.597 ..... 7059.66%
http/cluster.js type=bytes length=4 c=50: ./node-after: 17203 ./node-before: 14735 ............................... 16.75%
http/cluster.js type=bytes length=4 c=500: ./node-after: 15383 ./node-before: 15141 ............................... 1.60%
http/cluster.js type=bytes length=1024 c=50: ./node-after: 15554 ./node-before: 15133 ............................. 2.78%
http/cluster.js type=bytes length=1024 c=500: ./node-after: 14598 ./node-before: 14316 ............................ 1.97%
http/cluster.js type=bytes length=102400 c=50: ./node-after: 844.36 ./node-before: 887.37 ........................ -4.85%
http/cluster.js type=bytes length=102400 c=500: ./node-after: 760.55 ./node-before: 850.88 ...................... -10.62%
http/cluster.js type=buffer length=4 c=50: ./node-after: 14737 ./node-before: 15454 .............................. -4.64%
http/cluster.js type=buffer length=4 c=500: ./node-after: 13455 ./node-before: 13824 ............................. -2.67%
http/cluster.js type=buffer length=1024 c=50: ./node-after: 14033 ./node-before: 14390 ........................... -2.48%
http/cluster.js type=buffer length=1024 c=500: ./node-after: 14124 ./node-before: 14843 .......................... -4.85%
http/cluster.js type=buffer length=102400 c=50: ./node-after: 12275 ./node-before: 11311 .......................... 8.52%
http/cluster.js type=buffer length=102400 c=500: ./node-after: 11404 ./node-before: 12162 ........................ -6.23%
http/end-vs-write-end.js type=asc kb=64 c=100 method=write: ./node-after: 4567.7 ./node-before: 4516.2 ............ 1.14%
http/end-vs-write-end.js type=asc kb=64 c=100 method=end: ./node-after: 4582.2 ./node-before: 4417.7 .............. 3.72%
http/end-vs-write-end.js type=asc kb=128 c=100 method=write: ./node-after: 2688.4 ./node-before: 2932.7 .......... -8.33%
http/end-vs-write-end.js type=asc kb=128 c=100 method=end: ./node-after: 2985.8 ./node-before: 3066.6 ............ -2.63%
http/end-vs-write-end.js type=asc kb=256 c=100 method=write: ./node-after: 1733.4 ./node-before: 1703.8 ........... 1.74%
http/end-vs-write-end.js type=asc kb=256 c=100 method=end: ./node-after: 1805.5 ./node-before: 1656.4 ............. 9.00%
http/end-vs-write-end.js type=asc kb=1024 c=100 method=write: ./node-after: 361.9 ./node-before: 362.04 .......... -0.04%
http/end-vs-write-end.js type=asc kb=1024 c=100 method=end: ./node-after: 362.07 ./node-before: 357.1 ............. 1.39%
http/end-vs-write-end.js type=utf kb=64 c=100 method=write: ./node-after: 3483.3 ./node-before: 3697.7 ........... -5.80%
http/end-vs-write-end.js type=utf kb=64 c=100 method=end: ./node-after: 3986.4 ./node-before: 3818 ................ 4.41%
http/end-vs-write-end.js type=utf kb=128 c=100 method=write: ./node-after: 2320.3 ./node-before: 2177.1 ........... 6.58%
http/end-vs-write-end.js type=utf kb=128 c=100 method=end: ./node-after: 2532.2 ./node-before: 2437.6 ............. 3.88%
http/end-vs-write-end.js type=utf kb=256 c=100 method=write: ./node-after: 1388 ./node-before: 1411.2 ............ -1.65%
http/end-vs-write-end.js type=utf kb=256 c=100 method=end: ./node-after: 1622.8 ./node-before: 1367.6 ............ 18.67%
http/end-vs-write-end.js type=utf kb=1024 c=100 method=write: ./node-after: 412.43 ./node-before: 415.73 ......... -0.79%
http/end-vs-write-end.js type=utf kb=1024 c=100 method=end: ./node-after: 385.78 ./node-before: 399.51 ........... -3.44%
http/end-vs-write-end.js type=buf kb=64 c=100 method=write: ./node-after: 7876.5 ./node-before: 6939.5 ........... 13.50%
http/end-vs-write-end.js type=buf kb=64 c=100 method=end: ./node-after: 7916.3 ./node-before: 7418.2 .............. 6.71%
http/end-vs-write-end.js type=buf kb=128 c=100 method=write: ./node-after: 6000.9 ./node-before: 6474.2 .......... -7.31%
http/end-vs-write-end.js type=buf kb=128 c=100 method=end: ./node-after: 6715.9 ./node-before: 6786.8 ............ -1.04%
http/end-vs-write-end.js type=buf kb=256 c=100 method=write: ./node-after: 5026.9 ./node-before: 5400.1 .......... -6.91%
http/end-vs-write-end.js type=buf kb=256 c=100 method=end: ./node-after: 5380.5 ./node-before: 5861.6 ............ -8.21%
http/end-vs-write-end.js type=buf kb=1024 c=100 method=write: ./node-after: 2646.3 ./node-before: 2466.3 .......... 7.30%
http/end-vs-write-end.js type=buf kb=1024 c=100 method=end: ./node-after: 2518.6 ./node-before: 2407.1 ............ 4.63%
http/_chunky_http_client.js len=1 num=5 type=send: ./node-after: 64.799 ./node-before: 64.931 .................... -0.20%
http/_chunky_http_client.js len=1 num=50 type=send: ./node-after: 173.88 ./node-before: 187.61 ................... -7.32%
http/_chunky_http_client.js len=1 num=500 type=send: ./node-after: 243.19 ./node-before: 233.04 ................... 4.36%
http/_chunky_http_client.js len=1 num=2000 type=send: ./node-after: 239.9 ./node-before: 247.48 .................. -3.06%
http/_chunky_http_client.js len=4 num=5 type=send: ./node-after: 135.87 ./node-before: 161.15 ................... -15.69%
http/_chunky_http_client.js len=4 num=50 type=send: ./node-after: 176.15 ./node-before: 131.64 ................... 33.81%
http/_chunky_http_client.js len=4 num=500 type=send: ./node-after: 219.26 ./node-before: 237.87 .................. -7.82%
http/_chunky_http_client.js len=4 num=2000 type=send: ./node-after: 271.31 ./node-before: 224.63 ................. 20.78%
http/_chunky_http_client.js len=8 num=5 type=send: ./node-after: 120.67 ./node-before: 138.4 .................... -12.81%
http/_chunky_http_client.js len=8 num=50 type=send: ./node-after: 187.72 ./node-before: 202.46 ................... -7.28%
http/_chunky_http_client.js len=8 num=500 type=send: ./node-after: 222.44 ./node-before: 219.25 ................... 1.45%
http/_chunky_http_client.js len=8 num=2000 type=send: ./node-after: 275.45 ./node-before: 208.39 ................. 32.18%
http/_chunky_http_client.js len=16 num=5 type=send: ./node-after: 128.75 ./node-before: 114.76 ................... 12.19%
http/_chunky_http_client.js len=16 num=50 type=send: ./node-after: 169.7 ./node-before: 199.12 .................. -14.77%
http/_chunky_http_client.js len=16 num=500 type=send: ./node-after: 203.63 ./node-before: 208.51 ................. -2.34%
http/_chunky_http_client.js len=16 num=2000 type=send: ./node-after: 244.63 ./node-before: 235.49 ................. 3.88%
http/_chunky_http_client.js len=32 num=5 type=send: ./node-after: 104.66 ./node-before: 48.936 .................. 113.87%
http/_chunky_http_client.js len=32 num=50 type=send: ./node-after: 202.27 ./node-before: 162.8 ................... 24.25%
http/_chunky_http_client.js len=32 num=500 type=send: ./node-after: 242.88 ./node-before: 241.29 .................. 0.66%
http/_chunky_http_client.js len=32 num=2000 type=send: ./node-after: 239.01 ./node-before: 206.21 ................ 15.91%
http/_chunky_http_client.js len=64 num=5 type=send: ./node-after: 143.12 ./node-before: 48.008 .................. 198.12%
http/_chunky_http_client.js len=64 num=50 type=send: ./node-after: 222.88 ./node-before: 135.39 .................. 64.62%
http/_chunky_http_client.js len=64 num=500 type=send: ./node-after: 227.41 ./node-before: 240.4 .................. -5.40%
http/_chunky_http_client.js len=64 num=2000 type=send: ./node-after: 264.16 ./node-before: 251.04 ................. 5.23%
http/_chunky_http_client.js len=128 num=5 type=send: ./node-after: 46.779 ./node-before: 44.096 ................... 6.09%
http/_chunky_http_client.js len=128 num=50 type=send: ./node-after: 61.551 ./node-before: 188.66 ................ -67.37%
http/_chunky_http_client.js len=128 num=500 type=send: ./node-after: 217.13 ./node-before: 266.04 ............... -18.38%
http/_chunky_http_client.js len=128 num=2000 type=send: ./node-after: 232.96 ./node-before: 259.63 .............. -10.28%
http/simple.js type=bytes length=4 chunks=0 c=50: ./node-after: 9245.3 ./node-before: 9118.4 ...................... 1.39%
http/simple.js type=bytes length=4 chunks=0 c=500: ./node-after: 9255.5 ./node-before: 9152.4 ..................... 1.13%
http/simple.js type=bytes length=4 chunks=1 c=50: ./node-after: 8326.3 ./node-before: 8520.3 ..................... -2.28%
http/simple.js type=bytes length=4 chunks=1 c=500: ./node-after: 7902.2 ./node-before: 7738.2 ..................... 2.12%
http/simple.js type=bytes length=4 chunks=4 c=50: ./node-after: 1185.1 ./node-before: 1189.9 ..................... -0.40%
http/simple.js type=bytes length=4 chunks=4 c=500: ./node-after: 5658.5 ./node-before: 5339.8 ..................... 5.97%
http/simple.js type=bytes length=1024 chunks=0 c=50: ./node-after: 8493 ./node-before: 8849.1 .................... -4.03%
http/simple.js type=bytes length=1024 chunks=0 c=500: ./node-after: 8121.5 ./node-before: 8035 .................... 1.08%
http/simple.js type=bytes length=1024 chunks=1 c=50: ./node-after: 6813.1 ./node-before: 6005.2 .................. 13.45%
http/simple.js type=bytes length=1024 chunks=1 c=500: ./node-after: 6371.4 ./node-before: 7003.9 ................. -9.03%
http/simple.js type=bytes length=1024 chunks=4 c=50: ./node-after: 1189.9 ./node-before: 1191.7 .................. -0.15%
http/simple.js type=bytes length=1024 chunks=4 c=500: ./node-after: 4826.9 ./node-before: 5148.8 ................. -6.25%
http/simple.js type=bytes length=102400 chunks=0 c=50: ./node-after: 397.49 ./node-before: 435.99 ................ -8.83%
http/simple.js type=bytes length=102400 chunks=0 c=500: ./node-after: 410.8 ./node-before: 445.48 ................ -7.78%
http/simple.js type=bytes length=102400 chunks=1 c=50: ./node-after: 235.8 ./node-before: 251.15 ................. -6.11%
http/simple.js type=bytes length=102400 chunks=1 c=500: ./node-after: 254.27 ./node-before: 253.64 ................ 0.25%
http/simple.js type=bytes length=102400 chunks=4 c=50: ./node-after: 1953.3 ./node-before: 2025.7 ................ -3.57%
http/simple.js type=bytes length=102400 chunks=4 c=500: ./node-after: 1822.2 ./node-before: 1971.7 ............... -7.58%
http/simple.js type=buffer length=4 chunks=0 c=50: ./node-after: 7697.4 ./node-before: 8308.6 .................... -7.36%
http/simple.js type=buffer length=4 chunks=0 c=500: ./node-after: 7722 ./node-before: 7645.4 ...................... 1.00%
http/simple.js type=buffer length=4 chunks=1 c=50: ./node-after: 7423 ./node-before: 7111.4 ....................... 4.38%
http/simple.js type=buffer length=4 chunks=1 c=500: ./node-after: 7565.9 ./node-before: 7081.1 .................... 6.85%
http/simple.js type=buffer length=4 chunks=4 c=50: ./node-after: 7343 ./node-before: 6543.7 ...................... 12.21%
http/simple.js type=buffer length=4 chunks=4 c=500: ./node-after: 6732.3 ./node-before: 6857.2 ................... -1.82%
http/simple.js type=buffer length=1024 chunks=0 c=50: ./node-after: 8176.5 ./node-before: 8441.2 ................. -3.14%
http/simple.js type=buffer length=1024 chunks=0 c=500: ./node-after: 7648.5 ./node-before: 8213.4 ................ -6.88%
http/simple.js type=buffer length=1024 chunks=1 c=50: ./node-after: 8139.4 ./node-before: 7969 .................... 2.14%
http/simple.js type=buffer length=1024 chunks=1 c=500: ./node-after: 7342.6 ./node-before: 7223.7 ................. 1.65%
http/simple.js type=buffer length=1024 chunks=4 c=50: ./node-after: 6663.5 ./node-before: 7265.7 ................. -8.29%
http/simple.js type=buffer length=1024 chunks=4 c=500: ./node-after: 5862.8 ./node-before: 6467.6 ................ -9.35%
http/simple.js type=buffer length=102400 chunks=0 c=50: ./node-after: 6431.9 ./node-before: 6522.8 ............... -1.39%
http/simple.js type=buffer length=102400 chunks=0 c=500: ./node-after: 6000.1 ./node-before: 6609.7 .............. -9.22%
http/simple.js type=buffer length=102400 chunks=1 c=50: ./node-after: 6263.9 ./node-before: 6122.4 ................ 2.31%
http/simple.js type=buffer length=102400 chunks=1 c=500: ./node-after: 6340.6 ./node-before: 5743 ................ 10.40%
http/simple.js type=buffer length=102400 chunks=4 c=50: ./node-after: 5644.4 ./node-before: 5665.6 ............... -0.37%
http/simple.js type=buffer length=102400 chunks=4 c=500: ./node-after: 5786.2 ./node-before: 5589.8 ............... 3.51%

cc @trevnorris (we need performance WG)