[ppml] draft-ietf-ipv6-ula-central-02.txt use cases

> ..., ULA-C's entire purpose is to guarantee uniqueness.  DFZ providers will
> not have to fear a collision at all, only that the route may be filtered
> somewhere down your line.  A large number of businesses have been built on
> "pay your money and take your chances", and I think many people will route
> ULA-C's under that mantra.

we already know that it'll cross a lot of private peering AS boundaries.  but
as to whether it'll reach the DFZ reliably, i don't know, and does anybody care
at this stage, since any of us who want to filter it will have an RFC to point
to as our defense?