Problem reported by Ulla Theiss and diagnosed by Shane Hathaway.  The
long and short of it is that persistent objects refer to their
connection which refers to its cache which refers to the persistent
objects.  We can't let GC find the cycle because persistent objects
are extension class objects, and, thus, don't participate in GC.

Add an explicit clear() method that removes non-ghost objects from the
ring without going to all the trouble of ghosting them.  At some
point, they'll just get deallocated (which will ghostify them).

A cleared cache has non-ghost objects in the dict that are not in the
ring, so we need to add some checks for whether the object is actually
in the ring.