DHCPv6 server to delete elements of the local configuration that were deleted from the CB

The DHCPv6 server is able to periodically fetch new configuration elements and updated configuration elements from the configuration backend. However, it currently doesn't delete those elements of the local configuration that have been deleted from the database. The DHCPv4 server is already doing it so it is a matter of mostly copying the logic from the v4 server.

We also have to make sure that the MySQL v6 configuration backend sets the ids of deleted elements for option definitions, options, networks and subnets.