Use thread-safe maps to back lru caches

[luca992]

• Use stately ConcurrentMutableMap in DiskLruCache. fixes #66
• Use cache4k which supports Size-based eviction of the least recently used entry as the implementation behind our LruCache abstraction for all targets. cache4k is thread safe as it also uses stately internally. Removes our non-thread safe lru cache implementation. fixes #75