CB: Implement CQLConfigBackendDHCPv4
The CQLConfigBackendDHCPv4 class implements Config Backend for Cassandra as described in https://gitlab.isc.org/isc-projects/kea/wikis/designs/configuration-in-db-design
The CQLConfigBackendDHCPv4 class implements Config Backend for Cassandra as described in https://gitlab.isc.org/isc-projects/kea/wikis/designs/configuration-in-db-design