Removing chunks from the unload queue when performing chunk lookups is a costly activity.
It drastically slows down server performance as many methods call getChunkAt, resulting in a bandaid
to skip removing chunks from the unload queue.
This patch optimizes the unload queue to instead use a boolean on the Chunk object itself to mark
if the chunk is active, and then insert into a LinkedList queue.
The benefits here is that all chunk unload queue actions are now O(1) constant time.
A LinkedList will never need to resize, and can be removed from in constant time when
used in a queue like method.
We mark the chunk as active in many places that notify it is still being used, so that
when the chunk unload queue reaches that chunk, and sees the chunk became active again,
it will skip it and move to next.
Aikar