Need to back off better when oldest networkstatus isn't downloading right

dizum hasn't published a new networkstatus in the past five days. So clients are rightly calling dizum the oldest, and trying to fetch a new version. But there isn't a new enough one, so they keep trying, once per minute.

May 05 01:36:47.708 [info] router_set_networkstatus(): Not replacing
network-status from directory server "dizum" at 194.109.206.212:80
(published 2007-05-01 19:33:17); we have a newer one (published
2007-05-01 19:34:23) for this authority.

http://74.98.7.159:16012/Chatty_Tor.zip has more details. We need to make clients remember that the networkstatus they're about to fetch is the same one they fetched last time, and back off if it keeps failing.

The other piece of this bug is that apparently the network isn't converging on a single networkstatus from dizum. The fellow who reported this bug has one from "2007-05-01 19:34:23", but moria2 is currently serving one from "2007-05-01 19:33:17". Is this because moria2 only asks dizum for updates, and dizum is down, whereas some dir mirrors ask other places? For completeness: tor26 is serving one from "2007-05-01 19:31:33", moria1 is serving one from "2007-05-01 19:30:30", and lefkada has one from "2007-05-01 19:29:54".

Presumably a few lucky dir mirrors got the latest one from dizum before it croaked, but the dir mirrors are only asking the dir authorities, so it's not spreading.

[Automatically added by flyspray2trac: Operating System: All]