actually run disconnect handlers when a client disconnects

src/common/mainloop.rs

@@ -221,7 +221,7 @@ async fn client_recv_loop<S, R>(client_id: ClientId,
                     match err {
                         NetworkError::ClientDisconnected => {
                             trace!("[client disconnected] {:?}", client_id);
-                            server_sender.send(ClientAction::Disconnect(client_id));
+                            server_sender.send(ClientAction::Disconnect(client_id)).await;
                             break;
                         }
                         _ => {