Refine definition of CMSD-Static keys

[wilaw]

[Reporting on behalf of Nicolas] CMSD-Static is designed to carry keys whose values will not be modified during hop-by-hop transmission. The current definition is ambiguous in that it suggests that these keys should persist over multiple requests for the object with the same name, such as a manifest or playlist.

Proposition:
CMSD-Static: Keys whose values persist over multiple hops of a single request for the object.

Instead of
CMSD-Static: Keys whose values persist over multiple requests for the object.