Feature Request: Support Cleanup of Cache at X% Full

[MattMencel]

Is it possible to add a new environment variable such that if the filesystem storage reaches a certain percent used, it would expire the oldest cache items?

CLEANUP_FILESYSTEM_PERCENT

1. Prepare to save a cache
2. Check the storage percentage used
3. If above CLEANUP_FILESYSTEM_PERCENT purge the oldest item in the cache.
4. Save the new cache item

I could see this working separately or in parallel with CLEANUP_OLDER_THAN_DAYS.

[john-funk]

#142