Use preferred OR for nodes with routerstatus and microdesc too.

extend_info_from_node() used to use the primary OR port (i.e. IPv4)
unless the node had routerinfo. Now that we have IPv6 addresses in
microdescs we may want to use them.

Note that this patch changes using r->cache_info.identity_digest into
using node->identity. I count on these being well synchronised, or
things would break in other ways. Right?