Fix selection of directories with non-preferred address families

In legacy/trac#17840 (moved) in 0.2.8.1-alpha, we sometimes fail to fall back to directories with addresses in non-preferred IP families:

• we didn't identify relays that we could fall back to correctly;
• we incorrectly assumed that every node would have an IPv4 address~~ - this doesn't apply to bridges~~;
• we counted relays when we had already fallen back to non-preferred addresses.

This likely affected bridge clients with IPv4 bridges, and clients in small networks.