constructor, since it is ignored and there is no current plan to
support two caches with almost-but-not-quite-the-same arguments.

This change has effects in many files.  The Connection and DB don't
pass this argument and don't bother setting it explicitly when it is
reset via DB APIs like setCacheDeactivateAfter().  XXX These APIs
remain, since existing code may depend on them, but they have no
effect.

New policy in cPersistence.c: A Persistent object can't have its
_p_jar set or deleted once it is in a cache.  Persistent already
implemented this policy for _p_oid; it seems safer to do the same for
the jar.

Add ringlen() method to cache objects (implemented as cc_ringlen).
This returns the length of the doubly linked list of non-ghost
objects.  Same as len(cache.lru_items()), but more efficient.  Only
used for testing at the moment.

In ring_corrupt(), don't raise a new exception if one has already been
set.  The old behavior masked useful information about the original
error / traceback.