Address feedback on ControlFlow and FlowControl

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.

src/proto/ping_pong.rs

@@ -353,7 +353,7 @@ mod test {
 
     impl ReadySink for Transport {
         fn poll_ready(&mut self) -> Poll<(), ConnectionError> {
-            let mut trans = self.0.borrow_mut();
+            let trans = self.0.borrow();
             if trans.closing || trans.start_send_blocked {
                 Ok(Async::NotReady)
             } else {