doctor should check for clock skew on dir auths
In #25756 (moved) we discovered that it can actually produce a user-visible complaint when dizum has its clock wrong by 65 seconds.
Specifically, when the combination of skew from the dir auth and from the relay doing the fetch sum to more than 60 seconds, then if the relay decides to update its consensus at just the right time, the relay will get a warn message.
It would be great if doctor checked the Date: header on dir auth responses, and did something smart if it's not right enough.
Note that the Date header gets chosen when the dir auth begins to respond to the request, so if it takes 25 seconds to finish downloading the request, you shouldn't decide that the Date was wrong by 25 seconds.