ControlFlow::poll_window_update now exposes, effectively, a Stream of
WindowUpdates. Callers no longer poll on invidual stream IDs. To
accomplish this, FlowControl maintains a queue of pending remote stream
ids.
Improve/shorten naming throughout FlowControl.
FlowControlState::check_window has been added so that FlowControl is now
consistent in the face of stream-level flow control errors.
Connection now exposes the ControlFlow functions without exposing the
ControlFlow interface publicly.
