Fixed Uncontrolled Rift Spread

Switched rifts over to using link.childCount() to track spread instead
of the hasGrownRifts flag in the tile entity. The flag had the flaw that
if the rift was replaced by a block, the flag would reset and the rift
could spread again. I think I remember this being intended as punishment
for messing with rifts but it's a problem when combined with World
Thread farming.
Minor Adjustments to Rift Spread

Made a small correction to the condition on max ancestors. Also
thoroughly tested this code to ensure that it really does limit rift
spread. Currently, 2 initial rifts will cause 4 more rifts to be created
eventually.