ClientPreferIPv6ORPort not honoured when running with bridges

Since get_configured_bridge_by_routerinfo() returns the same bridge regardless of which routerinfo we pass to it, the first found (seemingly the one configured first in the config file).

One effect of this is that rewrite_node_address_for_bridge() will prefer whatever is listed first. Looking at ClientPreferIPv6 wouldn't make sense here since we come here once (only one routerinfo is received), unless we start calling it for every configured bridge. That doesn't seem like the right thing though.

Note that the idea of having ipv6_preferred as a property of the node_t is that we'd like to be able to change its value, per relay, when we learn about how we have been in touch with the relay.