add Connection::max_concurrent_send_streams (#513)

This PR adds accessors to `client::Connection` and `server::Connection`
that return the send stream concurrency limit on that connection, as
negotiated by the remote peer. This is part of issue #512.

I think we probably ought to expose similar accessors for other
settings, but I thought it was better to add each one in a separate,
focused PR.

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

src/server.rs

@@ -529,6 +529,19 @@ where
     pub fn ping_pong(&mut self) -> Option<PingPong> {
         self.connection.take_user_pings().map(PingPong::new)
     }
+
+    /// Returns the maximum number of concurrent streams that may be initiated
+    /// by the server on this connection.
+    ///
+    /// This limit is configured by the client peer by sending the
+    /// [`SETTINGS_MAX_CONCURRENT_STREAMS` parameter][settings] in a `SETTINGS`
+    /// frame. This method returns the currently acknowledged value recieved
+    /// from the remote.
+    ///
+    /// [settings]: https://tools.ietf.org/html/rfc7540#section-5.1.2
+    pub fn max_concurrent_send_streams(&self) -> usize {
+        self.connection.max_send_streams()
+    }
 }
 
 #[cfg(feature = "stream")]