BUG/MEDIUM: dns: Be sure to renew IP address for already known servers

BUG/MEDIUM: dns: Be sure to renew IP address for already known servers

When a SRV record for an already known server is processed, only the weight is
updated, if not configured to be ignored. It is a problem if the IP address
carried by the associated additional record changes. Because the server IP
address is never renewed.

To fix this bug, If there is an addition record attached to a SRV record, we
always try to set the IP address. If it is the same, no change is
performed. This way, IP changes are always handled.

This patch should fix the issue #841. It must be backported to 2.2.