http2: multiple style and performance updates

* move CHECK statements into DEBUG checks
* improve performance by removing branches
  * Several if checks were left in while the code was being developed.
    Now that the core API has stablized more, the checks are largely
    unnecessary and can be removed, yielding a significant boost in
    performance.
* refactor flow control for proper backpressure
* use std::queue for inbound headers
* use std::queue for outbound data
* remove now unnecessary FreeHeaders function
* expand comments and miscellaneous edits
* add a couple of misbehaving flow control tests

PR-URL: https://github.com/nodejs/node/pull/16239


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>