CB: Implement MySQLConfigBackendDHCPv4 class
The MySQLConfigBackend class implements Config Backend for MySQL as described in https://gitlab.isc.org/isc-projects/kea/wikis/designs/configuration-in-db-design
The MySQLConfigBackend class implements Config Backend for MySQL as described in https://gitlab.isc.org/isc-projects/kea/wikis/designs/configuration-in-db-design