We set aside a range of the object ID space for use by the server. This
allows the server to bind an object to an ID for a client and pass that
object to the client. The client can use the object immediately and the
server can emit events to the object immdiately.
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.
