In memory Key/Value store with LRU expire and concurrent access

Description

Items are stored in N sharded/bucketized HashMaps to improve concurrency. Every Item is always behind a RwLock. Quering an item will return a guard associated to this lock. Items that are not locked are kept in a list to implement a least-recent-used expire policy. Locked items are removed from that lru list and put into the lru-list when they become unlocked. Locked Items will not block the hosting HashMap.