Create a dirauth DoS response procedure

We have the technical ability right now to rapidly rotate up to n-1 of the directory authorities to new IP addresses and new intermediate keys, simply by updating torrc files of dirauths. So long as at least one directory authority remains listening on its old IP address and is aware of the other directory authorities' new locations, it should still be possible to both produce a consensus and distribute it to new clients.

We should clearly document this procedure so we can execute it quickly if a majority of the Tor directory authorities fall victim to a DoS or compromise.

We should also consider altering client bundles to ship with a reduced consensus or descriptor set of ultra high-uptime directory mirrors, so that in the future we can rotate all n directory authorities without issue.