Revert "Removing broken chunk caching from World. Fixes BUKKIT-5425"

This reverts commit 8ede1c33de16489da78ccc2ea5aec79bc2382f8c.
Readded the chunk cache in World:getChunkAt(int,int) that was used by the previous versions of MinetickMod. Further more empty chunks and null chunks will not be cached, and cache hits that have been marked as unloaded in the mean time (ones that are removed from the chunk map in ChunkProviderServer) will be ignored