Stop publishing/abandon individual resources

[fibann]

There is no way at the moment to stop publishing a single resource - only the whole discovery agent can be disposed.

Stopping a resource in the UDP implementation is trivial. OTOH room-based matchmaking services usually only drop a room when no one is in it anymore - they provide no way to close/hide a room forcefully from a client.

Maybe we can add a StopPublishing/Abandon method that only guarantees that the resource will be stopped/destroyed eventually.