http2: set js callbacks once

Make the http2 binding a bit more efficient by setting the callback
functions once when the module is loaded rather than for each
`Http2Session` and `Http2Stream`.

PR-URL: https://github.com/nodejs/node/pull/24063


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Note: Landed with one collaborator approval after PR
      was open for 18 days