If the application initialized resources in Websocket::onStart() this is the
place to free them.
This method is called right before the clients will be all automatically closed.
There is no need to call Endpoint::close() manually in this method.
If this method is a Generator it will be resolved as a coroutine before the server
is allowed to fully shutdown. Additionally, if this method returns a Promise the
server will not shutdown until that promise resolves.
