refactor Collector interface for lock-free parallelizability
parallelize TopScoreDocCollector
parallelize FacetsCollector
parallelize TopFieldCollector
fix javadoc warnings
parallellize DocSetCollector
setParallelized() may be called after previous invocations of subCollector()
Allow for customizable search strategies, adding a SerialSearchStrategy and ParallelSearchStrategy

moved the relevant code out of IndexSearcher into these strategy implementations