new 'extra dormant' mode for people who never use their tor

If you stop using your Tor as a client (and it doesn't have dirport or orport open), after an hour you'll stop fetching descriptors, but you'll continue to fetch consensuses.

Periodically we hear plans like http://wiki.debian.org/FreedomBox/DesignAndToDos that will involve installing Tor on things by default.

We should teach Tor that if a lot of time passes (e.g. 24 hours), it should stop fetching consensuses also. It should also remember in its state file that it's doing so, so the dormancy survives across reboots.