Yeah, that's likely to help Cake on weak multicores; even if we can't
actually make individual Cake instances concurrent, we should benefit from
running ingress and egress in parallel with each other.

And there's every chance we *can* make Cake run concurrently to some
extent, even if it's just enqueue with dequeue.  So you can then bring four
cores to bear on simple bidirectional traffic.

- Jonathan Morton