tor clients stop working if one v3 authority goes away for 3 hours

If one of the v3 authorities goes away, clients that use the v3 consensus will fail to have a live one, and will refuse to build circuits.

We should make clients tolerate this by using an older consensus if they've got it. I think "at most 24 hours old" is a fine estimate -- not so high that it will point to useless descriptors, and not so low that a bit of downtime on the part of the authorities will be a killer.

Of course, this comes with an anonymity downside: the directory mirrors can try to trick you into using an old consensus. Perhaps if we get an old consensus then we should go straight to an authority for the next try (or after a few tries), and then consider any one we get from an authority to be safe?

When there are 10 authorities and it becomes clear that a majority of them never vanish, we should then consider removing this feature, since the anonymity risk will then outweigh the "it works" benefit. But for now, I think 0.2.0.9-alpha clients will be much happier having this feature.

[Automatically added by flyspray2trac: Operating System: All]