IndexSearcher#search(Query,int) should operate on a shared priority queue when configured with an executor [LUCENE-8727]

If IndexSearcher is configured with an executor, then the top docs for each slice are computed separately before being merged once the top docs for all slices are computed. With block-max WAND this is a bit of a waste of resources: it would be better if an increase of the min competitive score could help skip non-competitive hits on every slice and not just the current one.

---
Migrated from [LUCENE-8727](https://issues.apache.org/jira/browse/LUCENE-8727) by Adrien Grand (@jpountz), updated Oct 01 2019

Start at IndexSearcher#search(Query,int) and trace how executor-backed slices compute and merge their top docs. Inspect the block-max WAND min-competitive-score flow across slices; done means a shared priority queue lets score increases from one slice help skip non-competitive hits in every slice.

Written by the indexing model from the issue text.