TS/HM: Created cachingKeyCollector
HM: Use OpenBitSet of lucene; Much faster for most operation according to

http://lucene.apache.org/core/4_3_0/core/org/apache/lucene/util/OpenBitSet.html
HM: Better comment..
HM: Removed test; untestable caching..
Merge branch 'caching-collector'