buffer: reapply 07c06672
Original commit message:
buffer: align chunks on 8-byte boundary
When slicing global pool - ensure that the underlying buffer's data
ptr is 8-byte alignment to do not ruin expectations of 3rd party C++
addons.
NOTE: 0.10 node.js always returned aligned pointers and io.js should
do this too for compatibility.
R=@trevnorris