Updating entity tracking and such.

This should address several issues. First, it addresses issue #527, which was a result of the chunks not unloading fast enough from our internal storage. Second, it addresses issues with tracking falling blocks, including the need to remove them after something odd happens, like falling out of the world. Finally, it helps reduce overall CPU usage by condensing entity tracking into a single variable, which cuts down on lookup times and helps alleviate the time needed to save or unload a world.