[PLAT-860] Limit filecache

[jerryblakley]

If a binding is taking up too much disk space with cached files, delete enough to get it under the new limit and also reset the sleep period before rescanning to a lower threshold. Normally we clean cache once a day and remove cached files which haven't been accessed in eight days. If a binding sees a lot of file accesses and the amount of disk space needed to cache them grows, it is possible that it's sleep can be set as low as 2 hours, and will attempt to clean cache on that schedule.
This is a best effort attempt to keep bindings from overusing disk space for cached files. While a binding is in its sleep phase waiting for the next cache cleanup, it can still in principle add enough files to overuse its disk space allotment.
This change will attempt to delete cached files until the amount of disk space used is 16GB or less.