Force plugins to queue chunks for unload. Fixes #1208

CraftWorld.unloadChunk attempted to unload a chunk immediately instead of
going through the standard ChunkProviderServer unload queue. This caused
various issues with mods such as IC2 during server startup if a plugin
attempted to call this method.
All chunk unload requests from plugins such as MultiVerse, will now be queued
and unload during the next world tick.