Adjust gNMI SetRequest Update handling for list keys + value payloads

[earies]

Clarify that an Update to a list member targeted in the Path cannot include those list keys in the value payload either as a direct descendant or ultimate leafref'd node.

Since you cannot modify the list keys on a targeted Path, disallow the contents altogether in the payload. At best, the same values could be specified but since the Path is unprefixed while JSON_IETF encoding currently remains prefixed, an association needs to be drawn from a schema set perspective.

In addition, this reduces pointless and redundant bytes on the wire.