IndexSearcher has the ability to search an index using multiple threads, but this behavior is an opt-in. Could we enable it by default so that:
 - Users experience better latency out-of-the-box.
 - Lucene performs better in comparative benchmarks against other engines that leverage multiple threads by default.

Note that this isn't totally straightforward to implement, as we'd need to share the threadpool across all index searchers and think of when to shut down this threadpool?

Migrated from LUCENE-10487 by Adrien Grand (@jpountz)