Modify subnet and shared network objects to hold information about unspecified values

The values of the Subnet and Network objects should be encapsulated in the OptionalValue template to be able to mark some of the parameters as unspecified if they are not set in the CB commands that set new subnet and shared network. See https://gitlab.isc.org/isc-projects/kea/wikis/designs/configuration-in-db-design#encoding-other-unspecified-parameters for the details.

Such change also requires that the subnet and shared network parsers are modified to mark values as unspecified if they are not explicitly set in the command.