Define CLASSES of Use Cases

[SpencerDawkins]

We really don't have a good way to describe whether a use case is a live media use case or an interactive media use case, and that's not helping us think about use cases and requirements.

We started off (in side meetings at IETF 112) collecting use cases, and describing attributes for them (for example, using specific values for one-way latency), and we've been removing attributes that were intended to help us make that distinction, without adding anything that helped to replace the attributes.

Based on IETF 116 discussions with @fluffy and @suhasHere, and private discussions with @fiestajetsam, I think I have a definition that's worth discussing.