iterate: follow CNAMEs in stub mode

iterate: follow CNAMEs in stub mode

There are two forwarder modes in the resolver - full forwarder,
and a stub mode. The full forwarder expects upstream to fully
solve the request (so the upstream must be a recursive resolver).

The stub forwarder mode is primarily useful for directing traffic
to a trusted resolver or authoritative (e.g. forward queries for
an internal zone). The upstream may not know the full answer to
the query, and may answer only from its authority. In that case
the resolver should follow the partially solved CNAME instead
of serving the partial answer.