http2: streamline OnStreamRead streamline memory accounting

* avoid consecutive decrement/increment session memory calls.
* only Resize the buffer when it is needed.
* flip `stream_buf_offset_` condition to the LIKELY case.

PR-URL: https://github.com/nodejs/node/pull/30351


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>