Fix for #4955 -- Race condition & memory leak in Puppet::Util

The Puppet::Util.sync method was not thread safe and also leaked memory. I'm
not certain, but I believe the first is ironic and the second is merely a bug.

This patch addresses the problem by 1) refactoring so the sync objects
are never returned (and thus no one can cache a reference to one) 2) adding
reference counting 3) deleting them when they are no longer needed 4) doing
the thread safty dance.

It wasn't the first (or even second) solution considered, but it's the one
that I was able to make work in a way that I'm convinced is correct. Its
main advantage is that it puts all the tricky bits in one place.