Lengthen the consensus validity interval

Right now, the 24 hour "build a consensus in this interval or die horribly" time is chosen completely arbitrarily. I know of no reason to not allow 48 or 72 hours.

See comments on #2681 (moved) for more discussion of this point. There's some discussion there of choosing an optimal time, but I suggest we just have some clients try 48 or 72 hours and see how it goes.

I'm boldly going with 72.

cc'ing arma because he remembers where the pain points are on the directory info download food chain, and mike because he has felt the pain.