Verify cache size accounting

I've eyeballed the cache size accounting looking for big discrepancies, so I can say it's at least mostly right over short-term usage. We need to do this in code though. I would suggest, at least while we're debugging, adding a signal handler for USR2 (or USR1, whatever), that makes Cache list it's dirs, sum up the actual sizes of objects stored, and compare that against its current stored value.