Add support for disabling conflict resolution to kea-dhcp-ddns
Implements part of #1386 (closed) #huge-sorry
This MR:
- Adds a new parameter, use-conflict-resolution, to NameChangeRequest messages
- Modify D2 to support adding and removing DNS entries without employing conflict resolution
A subsequent MR will add a DDNS configuration parameter, to kea-dhcp<4/6>