Move read logic into stream and remove prio logic.

This is part one of a multi-step plan to get priorization work right.

The end goal is to have streams sit in a loop, concurrently
buffering frames from the response writer, while sending on the
session channel whether they are ready to write to it. The session
loop then prioritizes streams rather than frames. This should make
prioritization much better, and uniform handling of streams will be
useful once I start implementing server push.