C2MS12 — Deprecate fields duplicated between C2MS Messages and the Message Envelope

With the advent of the C2MS Message Envelope, some tracking or meta-data fields exist in both the new Message Envelope and in the already-existing C2MS Message. For now, it is the case that we want to leave fields in the C2MS message and encourage migration toward Message Envelope usage. In a follow-on release, the duplicated fields should be deprecated in the C2MS Message.