Tor 0.3.x clients won't use Guard-flagged relays as Guards if they don't have V2Dir, throwing off consensus position weights

Starting in Tor 0.3.0, clients have additional constraints on what counts as a usable guard. Specifically, node_is_possible_guard() now demands node_is_dir(node) before it will consider node as a guard.

But the path position weights in the consensus are based on the Guard flag.

This inconsistency will lead to Guards that don't have the V2Dir flag being way underused, and Guards that do have the V2Dir flag being overused.