add FaultTolerance.BulkheadBuilder.enableVirtualThreadsQueueing()

[Ladicek]

This method enables bulkhead queueing for synchronous actions executed on virtual threads. The implementation is a simple two-semaphores bulkhead, which has in fact already existed as FutureThreadPoolBulkhead. This commit generalizes that class and leaves FutureThreadPoolBulkhead as a special case, nearly empty, for guarding asynchronous actions returning Future<V>.

Fixes #987
Closes #1015

[magicprinc]

I love it!

Only a tiny question of optimization:

Isn't AtomicReference<Thread> executingThread = new AtomicReference<>(Thread.currentThread()); an overkill?

You only read it, so it can be
final Thread executingThread = Thread.currentThread()?

[Ladicek]

Yeah, that's possible. I might have had some mutations there in the past, who knows 🤷 Removing the AtomicReference should be possible.