Handle session termination

[ktff]

Closes #197

This PR adds handling of session termination according to https://datatracker.ietf.org/doc/html/draft-ietf-webtrans-http3#name-session-termination with one exception noted in open questions.

Open questions

• Protocol requires that the endpoint must reset the send side of all of the streams associated with the session upon learning that the session has been terminated. For that we need to track open streams by storing some handle to them in Worker struct, but all approaches that I see are either not possible at the moment or seem too costly:
  * quinn::SendStream and quinn::RecvStream streams are not Clone so we can't store them directly. This seems to be intentional so it's unlikely for this to be changed.
  * quinn library doesn't expose a way to close a stream with just StreamId so we can't use them. It's maybe possible to get adequate changes into quinn library to enable this.
  * While it's possible to wrap quinn streams in something like Arc<Mutex<_>>, it seems costly.
  @BiagioFesta any idea on how to resolve this? (EDIT: We'll leave it outside this PR)

Todo

• Open issue for reset the send side of all of the streams associated with the session upon learning that the session has been terminated once this gets merged. Reset send side on session termination #242

[BiagioFesta]

Thank you for this amazing contribution!

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@BiagioFesta any idea on how to resolve this?

I don't see any solution on top of my head. As you already correctly mentioned, closing a stream requires mutable access to it, and the stream is "something" left to the application layer.
A chance is to leverage interior mutability with a Mutex, but I'd doubt it is worth it as the connection is closing anyway and streams are bound to that.

I will reason more about it, but definitely is something we can leave outside this PR

[BiagioFesta]

Can we polish a little bit commits history? (E.g. Finish means finish is a funny commit description, but not very talkative :) )