Order LeafReaderContexts by Estimated Number Of Hits [LUCENE-8788]

We offer no guarantee on the order in which an IndexSearcher will look at segments during a search operation. This can be improved for use cases where an engine using Lucene invokes early termination and uses the partially collected hits. A better model would be if we sorted segments by the estimated number of hits, thus increasing the probability of the overall relevance of the returned partial results.

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Migrated from [LUCENE-8788](https://issues.apache.org/jira/browse/LUCENE-8788) by Atri Sharma (@atris), updated May 24 2019

No file or test is named. Start by tracing how IndexSearcher visits LeafReaderContexts during searches, then determine how estimated hit counts and early termination should affect segment ordering; done means partial results are more relevant under the proposed ordering.

Written by the indexing model from the issue text.