Optimize database usage for reading the config DB table

The current implementation relies too heavily on the database for read operations and this is causing performance issues during start up (and probable elsewhere).

An improvement would be to query the entire contents of a database table initially, and run read operations against the in-memory data. This speeds up reads considerably, at the cost of increasing write operations because those have to be replicated for the in-memory cache. This solution also increases memory usage. But in some situations, both of these are worthy trade-offs.

For this task, we can implement this pattern for one of the tables that would benefit the most from this: core_config. In subsequent issues and after further research in MOBILE-3821, we can apply the same pattern to other tables.