State sync dependencies or Safe Entity Pointers

[ashoulson]

Handling for entities to refer to other entities. If the other entity hasn't been received yet, add a callback registration system to be notified when it has.

Note: For entity arrival callbacks, handle entity removal (I.e. The entity never arrived before it was called to remove, so kill the pending callback)

[ashoulson]

Entity events already do this. Could investigate for other containers.