Graceful handling for HTTP/2 GoAway frames.

[tomchristie]

When working with an HTTP/2 connection, a GoAway frame can be used to gracefully close the connection.
Our handling for this hasn't been working correctly, see encode/httpx#2112 and PrefectHQ/prefect#7442.

This was largely resolved by @zanieb's work on #679, which ensured that we at least properly push the connection into a closable state.

However, there's still some improvement to make here. When an HTTP/2 connection is closed with GoAway we'll currently raise exceptions for any requests that are made against the connection before we've seen the frame. Really we ought to gracefully retry any requests that are beyond the last_stream_id.