Implement Client::poll_ready (#21)

Client::poll_ready ensures that the connection is ale to to initiate a new request stream to the remote server.  When the server is at capacity, a task is stored to be notified when capacity is available.

src/proto/connection.rs

@@ -45,7 +45,7 @@ impl<T, P, B> Connection<T, P, B>
     }
 
     /// Returns `Ready` when the connection is ready to receive a frame.
-    pub fn poll_ready(&mut self) -> Poll<(), ConnectionError> {
+    fn poll_ready(&mut self) -> Poll<(), ConnectionError> {
         // The order of these calls don't really matter too much as only one
         // should have pending work.
         try_ready!(self.ping_pong.send_pending_pong(&mut self.codec));
@@ -55,6 +55,11 @@ impl<T, P, B> Connection<T, P, B>
         Ok(().into())
     }
 
+    /// Returns `Ready` when new the connection is able to support a new request stream.
+    pub fn poll_send_request_ready(&mut self) -> Poll<(), ConnectionError> {
+        self.streams.poll_send_request_ready()
+    }
+
     /// Advances the internal state of the connection.
     pub fn poll(&mut self) -> Poll<(), ConnectionError> {
         match self.poll2() {