Instances Memory Leak Fix

Simple fix to remove destroyed instances from RAM memory.

Instances are known to cause a memory issue. They remain in memory even after being destroyed. This is due to a problem with EntityHandlers not getting cleaned up, and holding references to the instance world forcing the world to remain in memory. This patch listens to the RemoveWorldEvent and ensures proper disposal of the instance.

This mod is targeted at server owners whose games rely on on-demand instance creation and deletion, and should significantly decrease memory-related crashes on your servers.