Add http2 support

[RatanShreshtha]

Having support for htttp2 would be nice, but that's not a priority right now, we can use reuests-core's h22 branch as a reference.

[njsmith]

It's also going to require some substantial rearchitecting, because the HTTP/2 protocol requires that you always have a background task processing every socket, regardless of whether the program is trying to work on it. This will require substantially expanding the backend logic. (Right now our backend interface doesn't provide a way to run background tasks at all!) And unfortunately I'm pretty sure the h22 branch just ignores this problem, so it's probably not very useful as a reference.