Change Connection traits to return futures

[Ruben2424]

Hi,
i experimented a bit with quinn 0.9 from #145 and GATs in the Quic Connection trait.
I changed the trait and implemented it for quinn. And i refactored the h3 implementation to use the new trait.
Of curse this is not finished yet. There are still some failing tests, I removed some code parts that need some refactoring and documentation is missing. But the examples mostly work.

The disadvantages of this:

1. Breaks MSRV
2. Changes the Traits
3. Some other disadvantages I didn't think of?

Advantages:

1. No Boxing required for the connection trait.
2. Less Poll methods and because of that a simpler code base.
3. Maybe some other advantages I didn't think of?

Open Questions:

1. Do we need a driver for server like the client have one to manage the control and qpack streams?
2. Should we change other traits to return futures as well?

I wanted to ask for your opinion before I continue to work in this. Is this the direction this project should go?

[Ruben2424]

@seanmonstar @eagr what do you think?

[Ruben2424]

Polling data from the control stream when accepting a new request like before didn\´t work well with the async methods. So I separated the accept logic so one task can manage the control stream while the other is accepting requests. The client is already doing a similar thing.
This fixed some tests. But there is still a lot of work to do.