brainstorm ways to let Tor clients use yesterday's consensus more safely

Right now Tor clients won't use a consensus that's 25 hours old. But if the directory authorities don't agree on a consensus for a day, things can go bad. We need to investigate other tradeoffs in this space than the one we've currently picked.

For instance: if you got your directory consensus info when it was valid, but you haven't been able to get any new consensus, perhaps you should be more forgiving about the timestamp on the consensus you have. That's a slightly different scenario than believing a new consensus that's 48 hours old.

Another option is just to change 24 to 48, which probably doesn't put clients at much greater harm, but gives us a lot more breathing room for mistakes.

The implementation side of this will be tricky, because we'll need to make sure that clients can handle descriptors that are 36 hours out of date too. We started implementing that feature several times, but I think we've never finished it.