Implementation of a cache with the LRU eviction scheme

1. The cache supports the following operations:
    - insert item
    - update item
    - remove items
    - remove all expired items
    - get the top N most frequently accessed keys
    
2.  A hash map is used for the lookup of keys, and a list for storing values

3. Recently accessed keys are moved to the beginning of the list and the cache has a maximum size of items it can hold. 

4. A cache miss results in an exception - the entry is read from the datastore and written to cache 

5. The least recently used cache eviction scheme is used to remove items from the cache