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%

PR-URL: https://github.com/nodejs/node/pull/26837


Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>