Item Cache: Store values in cache with underflow and overflow

This class can store and retrieve values in cache with support to underflow and overflow containers.

It can store a limited number of values with given keys in a cache array. When the cache size limit is reached, the class can call an external container overflow function to flush the oldest element in the class before it is removed.

The class can also retrieve values from the cache with given keys. When the requested value is not yet in the cache array, the class can call an external container underflow function to retrieve the value from a storage container.

A separate container class to store and retrieve values from a MySQL database.