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

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines