Nima: HTTP/2 robustness and production quality code

[tomas-langer]

Make sure we cover all that is needed:

• exception handling
• configurability
• robustness (make sure we do not consume too much memory, implement backpressure where needed etc.)
• fix all todos in code

Issues

• Nima - HTTP/2 Client #4910
• Nima - HTTP/2 Client - outbound continuation #6543
• Nima - HTTP/2 Client - trailing headers #6544
• Nima - HTTP/2 Client - validate protocol #6547
• Nima - HTTP/2 Client - upgrade failure fallback #6548
• Nima - HTTP/2 Server - trailing headers #7647

These issues have been deferred to a future 4.x release

• Nima - HTTP/2 Client - graceful connection closing #6545
• Nima - HTTP/2 Client - priority handling #6546

[Tomas-Kraus]

I can have a look at this.

[Tomas-Kraus]

Passing back to @tomas-langer for evaluation. Lot of things were already implemented, but this task is too common to be sure that everything is done.