Fallback Directory Selection should exclude down relays earlier
The updateFallbackDirs.py script uses OnionOO to find a list of candidate directory mirrors, then checks the consensus download speed from each mirror.
Previously, the script allowed relays that had a good uptime history, but just happened to be down right now.
But this doesn't work any more, because those relays can't provide a consensus, so we exclude them in the final consensus download check.
We could be smarter, and avoid the effort of that check by eliminating relays that aren't running right now from the list of fallback candidates.