TTL cache engine implemented

 - Added LRU_TTL cache engine that discards expired values. TTL is defined as time.Duration, not just int64, just to avoid confusion (is it ms? or ns? or something-else-ns?).
 - Switched dependant groupcache subpackages from "github" to local.
 - Switched groupcache to use lru_ttl instead of just lru
 - Tests were updated as well