⛱️ HTTP/2 Session Refactoring

[masaori335]

Overview

Change Http2Session and Http2Stream to have appropriate states as Continuation Handler instead of having a bunch of flags.

Motivation

For a long time, we've been struggling bunch of issues with the HTTP/2 subsystem. IMO, one of the root causes is Http2Session and Http2Stream are handling every event on only main_event_handler and handling many corner cases with flags.
Now, we're going to support the HTTP/2 Session on the origin server side too, this might make the situation worse.

/cc #7622

[masaori335]

Below is the first idea of states (v0.1).

HTTP/2 Session

• https://gist.github.com/masaori335/f16a851b50bec2f69e4034844a40ab59#file-http2ssn-uml
• HTTP/2 Session & ConnectionState Cleanup #7845 has piece of code for this

HTTP/2 Transaction

• https://gist.github.com/masaori335/f16a851b50bec2f69e4034844a40ab59#file-http2txn-uml
• We might want a state after signaling EOS or ERROR to the HttpSM with this.

[maskit]

Please keep #5675 in mind, although it doesn't need to be done on the first step of this.

[github-actions]

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