- use ShortObjMap with chunk coord keys instead of HashSet<PendingBlockUpdate>
  - deduplicate PendingBlockUpdate by coord
  - pool ShortObjMap and TreeSet objects

- removed unused variables
  - use ThreadLocalRandom instead of default world Random

Now we don't wait for finishing of writing all worlds. Waiting only for current world, then closing region files. Currently implemented for ImportChunkLoader only, should be implemented for default chunk loader in future

This reverts commit 3aa3ad9.
(Issues with some mods that uses random seed)

Do not rewrite json file if cache not changed, write it async

It is both CPU and memory optimization. Maximum index - 31, so array size is 32. It less than usual has table in TIntObjectHashMap

Newly populated chunk contains lots of PendingBlockUpdate and EntityFallingBlock, that takes lot of memory. PendingBlockUpdate processing causes more EntityFallingBlock spawning. In its turn, processing of EntityFallingBlock causes PendingBlockUpdate scheduling again. It solves by multiple iterations of processing both PendingBlockUpdate and EntityFallingBlock. Finally, we removing any EntityItem, that may be spawned at that time

Now we calling utilizeCPU method instead of Thread.sleep in in main server loop. In this method we can do something more useful than sleep. Currently implemented SyncServerExecutor queue processing and chunk generation