Clean up implementation and firing of both of these events by routing
both unload and load behaviors to consistent method calls.
This fixes issues where a few places would not call Load or Unload events
when it should have.
Additionally, reduces diff by moving the neighbor marking code into these
consistent points.
Additional benefits of the change include improving the neighbor marking
methods to use getChunkIfLoaded instead of getLoadedChunkAt in some places,
as the latter will cause chunks to be marked active and not unload.
Finally, this also updates CraftWorld.loadChunk to use the new methods, as the
previous logic did not properly handle the new unload queue.
