General Caching Query Runners cleanup (40% query time reduction for HLL)

* Add type strictness to CachingClusteredClient.
* Add background caching to CachingClusteredClient. Gives between 0% and 5% query speed increase.
* Add @BackgroundCaching annotation for injected ExecutorService items
* Add `numBackgroundThreads' configuration options to CacheConfig
* Add unit tests for CacheConfig
* Add an abstract caching query runner class, currently it doesn't do anything exceppt simply make the two caching queries distinct.
* Add caching to CachingQueryRunner. Gives up to a WHOPPING 40% reduction in query time on HLL queries
Yes, they look good to me.
TopN is the closest to not being thread safe. It is tread-safe so long as `query` is immutable.