src: store onread callback in internal field
This gives a slight performance improvement. At 2000 runs:
confidence improvement accuracy (*) (**) (***)
net/net-c2s.js dur=5 type='buf' len=64 *** 0.54 % ±0.16% ±0.21% ±0.27%
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
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