Don't fail to start up because a client or server doesn't resolve (DNS)

We probably shouldn't exit during startup just because we fail resolving a single peer (client or server).

See https://postlister.uninett.no/sympa/arc/radsecproxy/2011-11/msg00007.html for a discussion.

I think that a proper solution would be to postpone resolving a peer address until it's time to connect (servers) or verify a source address (clients).