This resolves the super common complaint about mobs not spawning.
This was ultimately a flaw in the vanilla count algorithim that allows
spawners and other misc mobs to count against the mob limit, which are
not bounded, and can prevent the entire world from spawning new.
I believe Bukkits changes around persistence may of actually made it
worse than vanilla.
This should fully solve all of the issues around it so that only natural
influences natural spawns.
Aikar