targeted cache invalidation

[umputun]

The current implementation is not any smart and invalidates cache entirely on any write/change.

some obvious places to make it smarter:

1. invalidate an affected site (multi-site configurations only)
2. vote/score change should invalidate post 's key(s) only
3. for edit & delete - invalidate post, last and user's keys
4. for pin - invalidate post's only

[umputun]

This was done with the scopes defining compound keys and flush via deletes. With the current go-cache it is impossible to get just the keys (without copying the cache map) and I had to maintain a parallel set of active keys and sync with go-cache via they eviction event. All of this seems to work ok.