BooleanWeight should decide how to execute minNrShouldMatch [LUCENE-4872]

#5637 adds a dedicated document-at-time scorer for minNrShouldMatch which can use advance() behind the scenes.

In cases where you have some really common terms and some rare ones this can be a huge performance improvement.

On the other hand BooleanScorer might still be faster in some cases.

We should think about what the logic should be here: one simple thing to do is to always use the new scorer when minShouldMatch is set: thats where i'm leaning.

But maybe we could have a smarter heuristic too, perhaps based on cost()

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Migrated from [LUCENE-4872](https://issues.apache.org/jira/browse/LUCENE-4872) by Robert Muir (@rmuir), 1 vote, updated May 09 2016
Parent: #5637
Attachments: [crazyMinShouldMatch.tasks](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-4872/crazyMinShouldMatch.tasks)

Start with the dedicated document-at-a-time scorer from #5637 and inspect how BooleanWeight currently chooses between it and BooleanScorer for minNrShouldMatch. Review the possible cost() heuristic and the crazyMinShouldMatch.tasks attachment, then define and validate a selection policy that is faster for both common and rare-term cases.

Written by the indexing model from the issue text.