Multi-Threading in IndexSearcher (after removal of MultiSearcher and ParallelMultiSearcher) [LUCENE-2840]

Spin-off from parent issue:

> We should discuss about how many threads should be spawned\. If you have an index with many segments, even small ones, I think only the larger segments should be separate threads, all others should be handled sequentially\. So maybe add a maxThreads cound, then sort the IndexReaders by maxDoc and then only spawn maxThreads-1 threads for the bigger readers and then one additional thread for the rest?

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Migrated from [LUCENE-2840](https://issues.apache.org/jira/browse/LUCENE-2840) by Uwe Schindler (@uschindler), updated May 09 2016
Parent: #3911

Start with the parent issue #3911 and the migrated LUCENE-2840 discussion. Clarify the intended threading model for IndexSearcher, including maxThreads, segment ordering, and how smaller segments are handled; the issue is not ready to define a concrete implementation or completion test.

Written by the indexing model from the issue text.