Least Recently Used (LRU) Eviction Strategy #57

Issue created from #53 (comment)

Currently the only eviction strategy is one of absolute time. If there was a method to define a "size" of cache, it would be worth exploring a "Least Recently Used" system to ensure that the hottest items stay in the cache.

Thoughts:

• Implement an extension (eg. AutoEvict/FixedCapacity) that manages the more advanced eviction strategy
• New extension tracks number of items in the cache, cache keys, expiry dates and last access dates
  • Could use a LinkedList to track most recently used or a timestamp directly
  • Probably easier with a ConcurrentDictionary with cache keys as the key and a custom type as the value
• Once a criteria has been hit (eg. X number of items in the cache), use what it knows to evict locally

Challenges:

• Keeping the extension up-to-date with cache data
  • Can use the various extension hooks but still is a bit fiddly
• Only evicting locally
  • Can do what I already do for RedisRemoteEvictionExtension (pass the cache layers in directly) but it is pretty cumbersome

Notes:
To handle a distributed LRU would be excessively complex and likely not useful. Would mean that every access to a cache item (including local) would have to then broadcast that access back through the cache system and whatever distributed backends are used.