Allow YRoom to be restarted and allow multi-room kernels

jupyter_server_documents/app.py

@@ -101,5 +101,5 @@ def _link_jupyter_server_extension(self, server_app):
     async def stop_extension(self):
         self.log.info("Stopping `jupyter_server_documents` server extension.")
         if self.yroom_manager:
-            await self.yroom_manager.stop()
+            self.yroom_manager.stop()
         self.log.info("`jupyter_server_documents` server extension is shut down. Goodbye!")