General Caching Query Runners cleanup

* 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 (default 0 aka same thread legacy behavior)
* 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
* Updated docs with more info on cache settings.
Merge pull request #936 from metamx/cachingRunnerImprovements

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