Fixed bug in garbage collector, which had a nasty habit
of activating inactive objects so that it could deactivate them.

activation.

undeactivatable.

if the cache size gets really big, the cache age can drop to as low as
20% of the configured cache age.  Also made the "minimize" method more
agressive.

minor, but who knows.

extend attr functions.

Fixed setattr bugs.

written.

make cache attributes changeable.

error.

from it.

Added _p_mtime attribute, which returns the persistent modification
time.

Added check in __save__ so that we don't save if we have decided that
we haven't changed.