If a queued job uses a waiter, then we want to run that waiter's
jobs in the current thread if all threads are busy, even if the
queue is empty, since there is no guarantee that any thread will
free up to take care of that new job, since all the threads might
be running a job which spawns such a recursive job and will block
till that recursive job is done, which it will never be since it
relies on the queue being polled by one of those blocked threads.
xmr-eric
moneromooo-monero
Howard Chu