Re-architect the cache mechanism to track max_size, max_age and cull_freq

[saurabh0719]

Description :

• Update the architecture to maintain an access list (using the LRU) to set default max_age of the cache and track max_age of each individual key-value pair.
• Also track the max_size of the cache and allow the user to set the default cull_freq (cull frequency - percentage of key-value pairs to be deleted when cache reaches it's max_size)
• Update cache (resolve max_age and LRU) after every operation. (set, get etc. etc.)
• Allow set() to take individual max_age for key-value pairs.