Fixed NPE in yCoordHelper, Other Bugs, Reimplemented Limbo Decay

I believe I've fixed the cause of a NullPointerException that some
players have been experiencing. It seems to have been caused by
unregistered block IDs being present in Limbo. Possibly because people
removed mods and they had generated structures in Limbo. Or the players
had placed blocks and then removed the mods, or changed block IDs
around.

This fixes issue #47.

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Also fixed a few other bugs around the code. I found that there were simple ways to circumvent Limbo decay as it was implemented, so I reorganized the code in CommonTickHandler to be more readable and wrote a new implemention of Limbo decay. Most of the logic is in its own class, LimboDecay. Please read the commit for more details.