Rewrote instance loading code, no longer requires destination world to be loaded. Instances no longer always spawn monsters. Instances have their own chunk generator which only generates empty chunks. Fixed bug with instance unloading happening whilst players are in the instance. Fixed bug with instance unloading never happening in some cases. Instance difficulty is now a setting on the portalpair, defaults to difficulty of the destination world at creation time. Optimized onPlayerMove to only perform checks when the player has crossed a block boundary.