(i hope)

Trouble here is that in pool_herder(), we access pp->dry unprotected, so
we might see an old value, thus we might breed more than wthread_min
even if the dry condition does not exist any more.

So for the vtc, we need to wait until wthread_timeout has passed and the
surplus thread has been kissed to death.

Notice that this does not change with #2942 because there the same
unprotected access happens to lqueue.