CB: Implement CQLConfigBackendDHCPv6
The CQLConfigBackendDHCPv6 class implements Config Backend for Cassandra as described in https://gitlab.isc.org/isc-projects/kea/wikis/designs/configuration-in-db-design
The CQLConfigBackendDHCPv6 class implements Config Backend for Cassandra as described in https://gitlab.isc.org/isc-projects/kea/wikis/designs/configuration-in-db-design