* Get ResourceStats before indexing
* Replaced localcache.CacheService to handle expiration faster (localcache.CacheService / gocache.Cache only expires values at specific interval, but we need to close index faster)
* singleflight getOrBuildIndex for the same key
* expire only in-memory indexes
* file-based indexes have new name on each rebuild
* Sanitize file path segments, verify that generated path is within the root dir.
* Add comment and test for cleanOldIndexes.