ConcurrentPool should not use the thread-local cache when running on a virtual thread

[lorban]

Jetty version(s)
12.0.x

Enhancement Description
ConcurrentPool has a thread-local cache that can be optionally enabled to increase the pool's performance for certain kinds of pooled objects. But this is counter-productive when the thread is a virtual one, so the pool should avoid using the cache in that case.

[lorban]

Recent benchmarks have shown that for Jetty 12, the concurrent pool's thread local cache tends to lower the performance when combined with the THREAD_ID strategy, which is the only time where the cache is enabled.

So it looks like the thread local cache should be removed, which would sort this issue out as a side effect.

[gregw]

THREAD_ID strategy is essentially a threadlocal hash anyway.

[gregw]

See fix in #11504

[lorban]

@gregw We already removed the thread local from ConcurrentPool as part of #10787 and deprecated the constructors that refer to it so I think we can close this issue.