Add the overflow queues again. We can easily iterate atomically over the
overflow queues and flush them.
Overflowing a queue is quite common when heavy swapping is done and
should never cause a lockup, so allocate new queues as we need them. We
can share the eventfd with the main queue to avoid wastings fds.
The limit on the number of queues is then only for when concurrent
threads want to invoke things, so 128 is plenty enough.
8c59fae42d