CB: Add two step configuration to the DHCPv6 server

Once the CfgMgr can merge local configuration with the configuration fetched from the database, the DHCPv6 server configuration and re-configuration procedure must be modified to first load configuration from the file and then from the database. The detailed state diagram is depicted here: https://gitlab.isc.org/isc-projects/kea/wikis/designs/configuration-in-db-design#configuration-state-diagram but this ticket excludes the bootstrap phase.