Generalize how IndexSearcher parallelizes collection execution

IndexSearcher takes an ExecutorService that can be used to parallelize collection execution. This is useful if you want to trade throughput for latency.

However, this executor service will only be used if you search for top docs. In that case, we will create one collector per slide and call TopDocs.merge in the end. If you use search(Query, Collector), the executor service will never be used.

But there are other collectors that could work the same way as top docs collectors, eg. TotalHitCountCollector. And maybe also some of our users' collectors. So maybe IndexSearcher could expose a generic way to take advantage of the executor service?