Move SCMP to CP draft

[nicorusti]

We went through the DP draft with Eliot at IETF121. He was overall positive, but mentioned that SCMP is a CP topic and (for now) move it to the CP

Ideas:

• [x] IS SCMP used anywhere by the control service? (@jiceatscion )
• SCMP is an additional signaling mechanism between endpoints and SCION routers (?). It could be a new section before security considerations in the CP draft
• [x] Add SCMP to the definitions in the CP draft
• [x] Add cross references from DP (handling link failures) to the CP draft

[jiceatscion]

I'll do the move, as I have a small fix to add to the DP anyway. Kevin or I can do a second pass to attend to the breadcrumbs

[jiceatscion]

Is SCMP used by the CS?:

• Yes!
• https://github.com/scionproto/scion/blob/dd852aa7a8e6eb99cb677851a7c55eb33bf64cb2/control/cmd/control/main.go#L217
• Link-down messages are used to revoke segments.
• The public goes "Ooooo"

[jiceatscion]

Move done per the PR The cross-referencing probably needs some more work.