replace L2 vanguards that aren't Fast or Stable

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.