resolve scary vanguard-related log msgs on dir auths

After we added layer-two vanguards, directory authorities wouldn't think any of their vanguards were suitable for circuits, leading to a "Failed to find node for hop #2 of our path. Discarding this circuit." log message once per second from startup until they made a fresh consensus. Now they look to their existing consensus on startup, letting them build circuits properly from the beginning.

Fixes bug 40802; bugfix on 0.4.7.1-alpha.