This method is invoked in a server connection when the client wishes to start an ``advise loop'' for the given topic and item by calling the start-advise method. The server can refuse to participate by returning #f. The client blocks until the server returns a value from this method.
The on-start-advise method is invoked with the topic used by the client to connect to the server and the item passed to the client's start-advise method.
Returns #t.