Previously there was always a likelihood that we would terminate our
retries if the mutator said that it was unable to be used, because
we had a number of mutators that were unconditional. However, now that
the mutators are able to be filtered, it is possible to select a
set of mutators which may always claim they are inappropriate. In
such a case, we would loop forever.

This change bounds the retries on looking for a mutator to 20
attempts - an arbitrary number I picked from the air as seeming
reasonable.