Added initial implementation for LRU cache

[Ernyoke]

This is a potential implementation of a simple LRU cache. It is not thread-safe yet.

It provides a basic interface which is a bit different than what the current implementation of the cache does. It could be refined as necessary.

It provides its own implementation of a doubly linked list. The reason for this is that the cache relies heavily of the inner workings of a double linked list.

[esimov]

Thank you very much for the huge work you've done with the LRU cache implementation. Very much appreciated.