sys/net/dhcpv6: discard stale replies
sys/net/dhcpv6: discard stale replies

The DHCPv6 server might send reponses multiple times.
The DHCPv6 client will only handle the first response, if additional
responses are comming in they are left in the RX queue.

That results in the client always reading the response of a previous
transaction on any subsequent transactions.
In this case the client will try again, creating a new transaction - that
will again only read the previous response.

To fix this, discard previous responses by flushing the RX queue before
sending a new message to the DHCPv6 server.

fixes #13834

OlegHahm/RIOT,miri64/RIOT,kYc0o/RIOT,kaspar030/RIOT,kYc0o/RIOT,authmillenon/RIOT,OTAkeys/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,jasonatran/RIOT,jasonatran/RIOT,kaspar030/RIOT,kaspar030/RIOT,kYc0o/RIOT,kaspar030/RIOT,OTAkeys/RIOT,authmillenon/RIOT,ant9000/RIOT,authmillenon/RIOT,kYc0o/RIOT,kYc0o/RIOT,ant9000/RIOT,ant9000/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,authmillenon/RIOT,miri64/RIOT,jasonatran/RIOT,authmillenon/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,ant9000/RIOT,jasonatran/RIOT,OTAkeys/RIOT,jasonatran/RIOT,miri64/RIOT,ant9000/RIOT,RIOT-OS/RIOT,miri64/RIOT,RIOT-OS/RIOT,miri64/RIOT