Reservation model

This is the abstract model used by ORCA brokers to return resource tickets to the SM controller. Each ticket contains a specifier for one or more slivers from a single AM named in the ticket. The SM controller obtains the slivers by redeeming these tickets to the AMs (redeem tickets). This model must describe the interdependency relationships among the slivers so that the SM controller can drive stitching for each sliver into the slice. Currently, ORCA uses the substrate delegation model: the ticket contains a resource type and unit count for the resources promised in the ticket, together with the complete original advertisement for the containing resource pool from the AM. NOTE: not yet implemented.