All messages are sent to connected clients by calling methods on the
Endpoint instance passed in onStart(). Applications must store
the endpoint instance for use once the server starts.
If the websocket application has external resources it needs to initialize
(like database connections) this is the place to do it.
If this method is a Generator it will be resolved as a coroutine before
the server is allowed to start. Additionally, this method returns a
Promise the server will not start until that promise resolves.
