may help #284
Cleans up the lighting queue system, reducing diff and improving implementation.
We no longer stop chunk unloads due to lighting updates, and instead simply flush the lighting queue.
The cost of forcing the chunk (and its neighbors!) to stay loaded waiting for its
lighting work to finish is much greater than simply taking the hit and doing the work.
This change also helps reduce the diff and avoid bugs with missed diffs by removing
duplicated logic.
Also switches to a more effecient data structure (ArrayDeque instead of LinkedList) for the queue itself.
