[PR] Support asynchronous merged-segment warming via a configurable executor [lucene]

["tlrx (via GitHub)" <gi...@apache.org>]

tlrx opened a new pull request, #16846:
URL: https://github.com/apache/lucene/pull/16846

   This is a draft PR for #16845.
   
   This makes merged segment warming able to run asynchronously off the merge thread, instead of always blocking it until warming finishes.
   
   The default behavior is unchanged, warming becomes asynchronous if you opt in by setting a merged segment warmer as usual and additionally provide an executor:
   
   ```java
     IndexWriterConfig iwc = new IndexWriterConfig(analyzer);
     iwc.setMergedSegmentWarmer(myWarmer);
     iwc.setMergedSegmentWarmerExecutor(myExecutor); // here
   ```
   
   With no executor configured (the default) warming stays synchronous and runs on the merge thread. 
   
   If the backing executor is saturated and rejects the task, the warming just falls back to running synchronously on the merge thread. The idea is to have some kind of back-pressure on merges rather than letting uncompleted/unwarmed merge work piling up unbounded.


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