http2: cleanup access via Symbols in core

Minimize property access via Symbols by consistently creating
new constants for things that are retrieved multiple times
within a method or a function. Fix cases where existing
constants aren't used where appropriate.

PR-URL: https://github.com/nodejs/node/pull/16327


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>