Also fixes some bugs in ours
Ultimately they both are near the same, but ours is behind the synchronized gate.
Mojangs is mixed behind 2 different synchronization contexts (chunks can lock 2 different objects)
Mojang also blindly unsets the ref on any chunk unload, not just if its the one being pointed to.
CraftBukkit removed their implementation that caused this issue,
switching to Mojang's implementation which doesn't appear to share it. I
already removed the important bit in the last upstream merge, this is
just unused and unnecessary now. So we remove it.
getting a loaded chunk is one of the most hottest pieces of code in the game.
Often, getChunkAt is called for the same chunk multiple times in a row, often
from getType();
Optimize this look up by using a Last Access cache.
