Request that the handler is unregistered from the server.
After the handler is unregistered, the handler's onClose method
will be called.
A handler can only be unregistered once. A given instance will return the same Promise if this method is called more than once.
a {@link Promise} that completes when a response is received from the server
A reference to a registered handler.
Such a handler reference is provided whenever a handler with a server side presence is registered.