Change mob spawner code to use Metadata instead of WeakHashMap

Unfortunately profiler data complains about memory usage from this
plugin even though it's all Weak data that's being stored internally by
the Bukkit anyways, which means the data is being counted twice.  This
should shift the burden away me here so users don't complain about what
they don't understand.  This probably causes a small leak when people
/butcher, but it's a small price to pay, and at that point it won't be a
problem on my end.