Consider removing fallback directory weights
We don't currently weight requests to authorities.
While the fallback directory code uses the weights, we could assign each fallback the same weight, or leave the weights out, and have them assigned the default weight.
If we didn't weight fallback directories, this would potentially improve:
- the statistics calculations on authorities,
- the privacy implications of high-weighted fallbacks seeing too many client requests.
(Implementing this ticket would obsolete #17888 (moved).)