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/state.rs

@@ -90,7 +90,8 @@ impl StreamState {
                 if eos {
                     *self = HalfClosedRemote(local);
                 } else {
-                    *self = Open { local, remote: Data(FlowControlState::with_initial_size(initial_recv_window_size)) };
+                    let remote = Data(FlowControlState::with_initial_size(initial_recv_window_size));
+                    *self = Open { local, remote };
                 }
                 Ok(true)
             }
@@ -300,6 +301,14 @@ impl StreamMap {
         self.inner.get_mut(&id)
     }
 
+    pub fn local_flow_controller(&mut self, id: StreamId) -> Option<&mut FlowControlState> {
+        self.inner.get_mut(&id).and_then(|s| s.local_flow_controller())
+    }
+
+    pub fn remote_flow_controller(&mut self, id: StreamId) -> Option<&mut FlowControlState> {
+        self.inner.get_mut(&id).and_then(|s| s.remote_flow_controller())
+    }
+
     pub fn has_stream(&mut self, id: StreamId) -> bool {
         self.inner.contains_key(&id)
     }