Can we use guards when we don't have a usable NetDir?

Currently, the path generation code will never use the guard manager for directory guards, unless it already knows a NetDir. But that's not necessarily so smart: maybe instead we should instead connect to the guards that we have, and use them to bootstrap the network.

Possible challenges:

• Is there a maximum "age" on our guard sample that we should enforce here, before we just use fallbacks instead?
• Should we mark all guards as possibly-usable after bootstrapping in this case?