[Reviewer: Rob] Refactor the cache to use the new generic cassandra store. 

This goes along with https://github.com/Metaswitch/cpp-common/pull/154. It mainly consists of the following changes:

* Delete a bunch of code that was already in `cassandra_store.cpp`
* Move a bunch of test code to cpp-common. 
* Make the cache fir with the new store objects (mainly renaming and changes to function prototypes). 

Tested with homestead UTs.