JGRP-2851 Use thread factory for virtual threads

[cfredri4]

This change makes ThreadPool use the supplied ThreadFactory also for virtual threads, which means they will have correct names etc

[belaban]

OK, I don't really care about JDKs < 17. But since 17 is LTS (IIRC), I'd like to support JDK 17 and higher. Your code searches for Executors.newThreadPerTaskExecutor(Factory), which is fine in JDKs > 17, but I cannot find this method in JDK 17.
Thoughts?

[cfredri4]

JDK 17 is LTS, but virtual threads are only available in Loom early access builds for JDK 15/16/17/18 (and as preview feature from JDK 19).
The code previously supported Loom early access builds so I tried to replicate that. But maybe support for Loom early access builds can be dropped and instead virtual threads only need to be supported for real builds (>=JDK19)? This would make ThreadPool+ThreadCreator simpler.

[belaban]

I agree that virtual thread support doesn't need to be in JDKs in which they were preview features. Nobody would run with preview enabled in production...

[cfredri4]

Rebased+pushed with simplifications to only support real builds.

[belaban]

Applied