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
* Updated docs with more info on cache settings.
the comment was about the need for mergeSequencesUnordered instead of mergeSequences.
It should be possible to use mergeSequences when the query intervals for two different node do not overlap.