Changes to Rift Regeneration

Moved a little of the regeneration code to BlockRift to reduce duplicate
code. Changed RiftRegenerator to iterate over loaded worlds instead of
all worlds. Removed forced rift generation call in
EventHookContainer.onWorldLoad() - I feel it would have minimal
benefits. Rifts now drop World Thread upon regeneration. Generally
cleaned up the code in FastRiftRegenerator and RiftRegenerator.