investigate puma/nginx socket activation

[pushcx]

https://github.com/puma/puma/blob/master/docs/systemd.md

The Puma docs describe two configurations. We implemented the first, simply listening to a local port. But socket activation sounds nice:

this keeps listening sockets open across puma restarts and achieves graceful restarts, including when upgraded puma

So the to-do here is to update our configs to this and hopefully switch to it.

[pushcx]

Puma now refers to this features as phased restarts. I got it working in 14a5cbc95, though not without some hassle that may lead to more changes. In any case, though, this is in good shape.