[WIP] Handles

[jedediah]

This is a generalization of the idea behind some recent SportBukkit changes

Objects that are prone to leakage, such as World, Entity, etc, can provide a Handle to themselves, a lightweight object that knows how to retrieve the object, but does not hold a strong reference to it. Trying to access the object beyond its natural lifespan throws an exception. Handles to the same object compare equal to each other, and this can be determined without dereferencing the object. A handle can also provide a dynamic proxy for the object that delegates method calls through the handle.

Objects can provide Handles through the Handleable interface. This uses a WeakReference by default, but this has the limitation that any leak of the object prevents the release of all Handles. Things like World or Entity would implement Handles using their UUID.

This needs a bit more work, and some implementation in SportBukkit and BungeeCord.