Currently DrillSideways implementation is based on the single threaded IndexSearcher.search(Query query, Collector results).
On large document set, the single threaded collection can be really slow.
The ParallelDrillSideways implementation could:
1. Use the CollectionManager based method IndexSearcher.search(Query query, CollectorManager collectorManager) to get the benefits of multithreading on index segments,
2. Compute each DrillSideway subquery on a single thread.