dnsdist: Check the flags to detect collisions in the packet cache

dnsdist: Check the flags to detect collisions in the packet cache

In the unlikely but quite real event two queries with the same qname,
qtype and qclass but different EDNS options or flags end up with
the same hash, the packet cache would return a answer that might
not be suitable for the query. Reduce the odds by checking the
flags present in the dns header in addition to the qname, qtype
and qclass.
For the same reason we might need to consider storing the ECS
subnet if any.