NewLRUCache - create lru cache `bucketCnt` is buckets that shard items to reduce lock racing `capPerBkt` is length of each bucket, can store `capPerBkt * bucketCnt` count of items in Cache at most optional `expiration` is item alive time (and we only use lazy eviction here), default `0` stands for permanent.

ToInt64 - convert bytes to int64.

Cache - concurrent cache structure.