Cache in CachingBlobStore might grow beyond configured limit

It appears that depending on actual cache entry sizes, the CacheLIRS might grow beyond the configured limit.

For RDBBlobStore, the limit is currently configured to 16MB, yet storing random 2M entries appears to fill the cache with 64MB of data (according to it's own stats).

The attached test case reproduces this.

(it seems this is caused by the fact that each of the 16 segments of the cache can hold 2 entries, no matter how big they are...)