BlockDiskCache partial remove / group remove doesn't work

BlockDiskCache implementation improperly handles removal using prefixes and groups.

I'm attaching test cases (mostly - copy&paste from IndexedDiskCache for testing of removals) and patch (mostly - copy&paste from IndexedCache).

I tried to extract that to super-class methods, but in case of tests - that would require opening API in AbstractDiskCache (most of the methods used in tests are protected), and in case of fix - that would probably need to also abstract the locking mechanism.