LUCENE-5299:
  factor out the core of IndexSearcher.search() into customizable SearchStrategy API. Default if none is supplied is SerialSearchStrategy.
  IndexSearcher constructor does not take an ExecutorService anymore, the previous users supply ParallelSearchStrategy is applicable
  the previous parallelization of TopScoreDocCollector & TopFieldCollector from within IndexSearcher has been removed
Make the TopDocsCollector's parallelizable
Make TotalHitCountCollector parallelizable
Make Solr's DocSetCollector parallelizable
Make Solr's EarlyTerminatingCollector parallelizable
parallelize TimeLimitingCollector
Make the anonymous SimpleCollector's in SolrIndexSearch parallelizable, created a new TopScoreCollector