World create fix?

World cache cant use weak refs, these are only refs to the worlds and
they contain state that can't be recreated if the CraftWorld gets GC'd
Merge remote-tracking branch 'origin/master' into world_mgt
Merge remote-tracking branch 'origin/master' into world_mgt
Both events needed?
Merge remote-tracking branch 'origin/master' into world_mgt
Merge pull request #410 from keepcalm/world_mgt

Merge world_mgmt - improves multiworld support. But you can't tp there yet.