Non blocking ReservedThreadExecutor (#6535)

A call to offer must never block, nor even yield, since to do so give an opportunity for the allocated CPU core to change, defeating the whole purpose of the class.
There is also some reasonable level of diagnostic warnings if a reserved thread misses too many offers consecutively, based on tracking the state of the reserved thread.

Remove the stack data structure entirely.  ReservedThreads all poll the same SynchronousQueue and tryExecute does a non blocking offer.

Added test for busy shrinking

Remember last time we hit zero reserved threads

Co-authored-by: Simone Bordet <simone.bordet@gmail.com>