We need to make sure the client doesn't reuse an object ID until the
server has seen the destroy request. When a client destroys an ID
the server will now respond with the display.delete_id event, which lets
the client block reuse until it receives the event.
So obvious in retrospect. The object system can do all the work for us
and keep track of pending calls as regular objects and we don't need to
abuse the resource system to get them cleaned up on client exit. We
don't need the custom key management or (broken) lookup, we just sue
object IDs. And last but not least, anybody can receive the callback,
not just display listeners.
