http2: improve http2 code a bit

Multiple general improvements to http2 internals for
readability and efficiency

[This backport applied to v10.x cleanly but had several
merge conflicts on v8.x.]

PR-URL: https://github.com/nodejs/node/pull/23984


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Ujjwal Sharma <usharma1998@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>