Implement `last_time_on_internet` for tor-guardmgr

guard-spec has an unimplemented feature:

    * {tvar:last_time_on_internet} -- the last time at which we
      successfully used a circuit or connected to a guard.  At
      startup we set this to "infinitely far in the past."
...
        Then check whether the {last_time_on_internet} is more than
        {INTERNET_LIKELY_DOWN_INTERVAL} seconds ago:

           * If it is, then mark all {PRIMARY_GUARDS} as "maybe"
             reachable.

At first I was thinking that we should implement this with something like #179 (closed), but I think that we might not actually want to do that. By the time we notice that a guard is successful, we've already read some cells on the internet, or else we wouldn't have noticed that the guard is successful.