Readd chunk saving queue

[Niko-sk2x]

Writing chunks to NBT can take quite a while, add a queue for that so that chunk unloading doesn't stall the main thread when something decides to unload too many chunks at once. Also related to #7

[Cyclonit]

Just a quick mention: This must be done in the main thread, otherwise concurrency issues will arise.

[Niko-sk2x]

Yes, but a queue can smooth the writing to NBT over many things for arbitarrily long time instead of what the code that asked for saving does.