CloudCacheStore should use metadata to store expiration information for more efficient purging

CloudCacheStore currently uses the BucketBasedCacheStore facilities to encode expiry information into the buckets. This means buckets need to be loaded (expensive) before such information can be read. It would be more efficient to encode this as metadata so that it can be read before potentially loading large blob values, and if the entry has expired, simply delete the entry rather than load it.