Message Management API for Peer Channels will not be accessible via the "Master Bearer Token"

[AustEcon]

• This will bring the messaging API fully back into line with the standard Peer Channels API - i.e. I don't think we should add the extension of allowing a master bearer token to access all channels.

Why?

• It complicates the implementation potentially all the way down to the db schema and requires more db look-ups on the hot path of message reads and writes.
• Every use case is already catered to without "Master Bearer Token" read/write access to the messaging APIs.

I am open to reconsidering this feature but there is so much to do - I don't think it's worth the detour for now...