Cleanup of stale cache entries

[joeytwiddle]

Just a placeholder to discuss this feature. :)

A metric I used in the past to order cache entries for cleanup was:

timeSinceLastHit / (numberOfTimesHit + 1)

So entries which are hit twice as often will take twice as long to expire. (The +1 just avoids division by zero, if we don't record the creation of the cache entry as a "hit".)

Although for this project, a fixed timeout like "1 month old" for the disk-cache might be simpler, and avoid having to record hit statistics. "1 month unused" might be better, but we would need to record the last hit time.