Drop AsyncWrite usage in h3-quinn once GATs land

[stammw]

We were forced to use AsyncWrite in h3_quinn::SendStream because Quinn only offers lifetime-bound Futures to poll for writes:
Write<'a, S>.

Generic associated type could cancel this limitation, allowing to store them into h3_quinn::SendStream.

This will have positive implication for performance and will remove the necessity to Pin the stream and to downcast the error type from IOError to quinn::WriteError.

[Ralith]

quinn could also be modified to expose raw poll_* style methods alongside important futures, if a better approach is required in the mean time. That's assuming it's not practical to modify h3 to push the pinning/lifetime issues downstream.

[stammw]

I remember that I thought of using the poll_* methods, but can't recall why I didn't ask for them to be made available.
I guess this solution will cancel the need of GATs whatsoever.

[inflation]

This should be closed after quinn 0.9.0 upgrade.