* Fix World#isChunkGenerated calls
Optimize World#loadChunk() too
This patch also adds a chunk status cache on region files (note that
its only purpose is to cache the status on DISK)
* Ensure correct regionfile usage
This also bumps the minimum region file cache to 4 files given
readChunkData can load potentially 4 files.
* Fix closed check
* Better checks for invalid regionfile usage
