Sort-based skipping and concurrency

Hi all,

I've been working on improving Sort skipping by using segment re-ordering[1] for sorted segments, which on benchmarking was working brilliantly on the wikimedium10k dataset, but then showing no improvements at all on wikimediumall  - very confusing!  On digging further, I found the issue to be with search concurrency.  The segment re-ordering happens within a single unit of work, but for sufficiently large segments we generate large numbers of tasks even if the concurrency is low - e.g., with an ExecutorService with 2 threads, we still get 17 tasks.

For score-based searches we share competitive scores between tasks using a MaxScoreAccumulator, but we don’t have anything similar for field-sort-based searches.  So these 17 tasks all run without any knowledge of what has happened before, and even if we re-ordered the tasks so that the first ones contained the best hits, the subsequent tasks wouldn’t be able to early terminate.

I can look into adding some support for sharing competitive value thresholds between threads, but I wonder if it’s worth considering merging the skipping infrastructure between score-based sorting and field-based sorting?

- Alan

[1] https://github.com/apache/lucene/pull/15436