Allow object caches to auto-clean unused keys.

[Zoramite]

Currently when using object caches (in core or extensions) that write to a file the process of removing old cache keys is manual. This makes it easy for the object cache to bloat over time.

One idea to correct this is to allow the object cache to be flagged for auto-cleanup. Then, whenever a full build is completed (not partial builds) any unused keys in an object cache can be removed before the cache file is written.