o Major bugfixes (vanguards):
    - Rotate to a new L2 vanguard whenever an existing one loses the
      Stable or Fast flag. Previously, we would leave these relays in the
      L2 vanguard list but never use them, and if all of our vanguards
      end up like this we wouldn't have any middle nodes left to choose
      from so we would fail to make onion-related circuits. Fixes bug
      40805; bugfix on 0.4.7.1-alpha.