ClientUseIPv4 0 tells tor to avoid IPv4 client connections.
ClientPreferIPv6DirPort 1 tells tor to prefer IPv6 directory connections.
Refactor policy for IPv4/IPv6 preferences.
Fix a bug where node->ipv6_preferred could become stale if
ClientPreferIPv6ORPort was changed after the consensus was loaded.
Update documentation, existing code, add unit tests.
