properly rotate mirrors out of rotation when rebooting

In #41734 (closed), we retired mininag because we were under the impression it wasn't serving us so well, particularly because we weren't respecting the required delays for servers to go out of rotation.

In fact, even with mininag, and even with higher delays (our procedures talk about 30 minutes shutdown delay), we were not doing this properly. The TTL on www.torproject.org is one hour, which means we'd need to wait at least that for records to expire properly.

So let's just review all of this stuff. Consider lowering that TTL to 10 or 20 minutes so that we can wait less long.

Then implement something in the reboot procedure to properly wait for that delay, and post something in autodns so that servers are taken out of rotation. fabric-tasks!3 (closed) is the start of that, but has many issues.

Alternatively, we could also rewrite mininag to talk to Prometheus as well, but, as discussed in #41734 (closed), that requires improvements to the node exporter that haven't landed yet.