CB: Add capability to merge DHCPv6 configuration from database and from a file

Part of the Config Backend feature is to change the logic of the DHCPv6 server during startup or reconfiguration to first read the partial config from a file and then connect to the database and fetch the rest of the configuration. Both configurations have to be merged into a single configuration. This ticket covers such a merge of the data fetched from the database into the CfgMgr. It doesn't cover the changes in the server logic to trigger such merge. This will be done in a separate issue.