Align MessageLayer under lib/message with current MessageHeader implementation under transport

[yufengwangca]

Problem
Currently WML baseline was ported with headers-only for dependencies, one of which is the Weave Message Layer.

The Weave Message Layer defines a common format for all Nest Weave messages, however it uses the weave defined methodology.

Proposed Solution
Align current Message Layer message format with agreed format used in current transport header definition and field handling implementation.

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