This fixes it, based on an approach suggested by Toby
Dickenson.  The triggering condition wasn't entirely
sane, so was very rare:  a persistent object with a
__del__ method that referenced an attribute of self.

scan_gc_items():  Look at persistent objects accessed
while this is running at most once.

New test checkMinimizeTerminates():  This spawns a thread
that will in fact run forever if you don't recompile
cPickleCache.c.  The test suite will keep running, but
checkMinimizeTerminates will fail, and all future calls
to cacheMinimize() will be effectively ignored (so other
bad things may happen as a result).