BlockMaxConjunctionScorer's simplified way of computing max scores hurts performance [LUCENE-8759]

BlockMaxConjunctionScorer computes the minimum value that the score should have after each scorer in order to be able to interrupt scorer as soon as possible. For instance say scorers A, B and C produce maximum scores that are equal to 4, 2 and 1. If the minimum competitive score is X, then the score after scoring A, B and C must be at least X, the score after scoring A and B must be at least X-1 and the score after scoring A must be at least X-1-2.

However this is made a bit more complex than that due to floating-point numbers and the fact that intermediate score values are doubles which only get casted to a float after all values have been summed up. In order to keep things simple, BlockMaxConjunctionScore has the following comment and code

```Java
// Also compute the minimum required scores for a hit to be competitive
// A double that is less than 'score' might still be converted to 'score'
// when casted to a float, so we go to the previous float to avoid this issue
minScores[minScores.length - 1] = minScore > 0 ? Math.nextDown(minScore) : 0;
```

It simplifies the problem by calling Math.nextDown(minScore). However this is problematic because it defeats the fact that TopScoreDocCollector calls setMinCompetitiveScore on the float value that is immediately greater than the k-th greatest hit so far.

nextDown(minScore) is not the value that we need. The value that we need is the smallest double that converts to minScore when casted to a float, which would be half-way between nextDown(minScore) and minScore. In some cases this would help get better performance out of conjunctions, especially if some clauses produce constant scores.

MaxScoreSumPropagator#setMinCompetitiveScore has the same issue.

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Migrated from [LUCENE-8759](https://issues.apache.org/jira/browse/LUCENE-8759) by Adrien Grand (@jpountz), updated Apr 16 2019
Attachments: [LUCENE-8759.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-8759/LUCENE-8759.patch)

Start with BlockMaxConjunctionScorer's minimum-score calculation and MaxScoreSumPropagator#setMinCompetitiveScore, then inspect the related competitive-score handling and existing tests. Reproduce or benchmark conjunction scoring with floating-point thresholds, and verify that the computed bounds preserve correctness while allowing the intended early interruption.

Written by the indexing model from the issue text.