Start converting the message parser to use the new ibuf api.

Start converting the message parser to use the new ibuf api.

Rewrite rde_update_dispatch() to use ibufs. Because of this
rde_update_err(), rde_get_mp_nexthop(), nlri_get_prefix() and
friends are switched to use ibufs. For rde_attr_parse() a minimal
change was done for now.

OK tb@