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/proto/connection.rs

@@ -120,6 +120,12 @@ where
         self.settings.send_settings(settings)
     }
 
+    /// Returns the maximum number of concurrent streams that may be initiated
+    /// by this peer.
+    pub(crate) fn max_send_streams(&self) -> usize {
+        self.streams.max_send_streams()
+    }
+
     /// Returns `Ready` when the connection is ready to receive a frame.
     ///
     /// Returns `RecvError` as this may raise errors that are caused by delayed