Add FanOut Message Delivery

[gabheadz]

Is your feature request related to a problem? Please describe.
No

Describe the solution you'd like
At the moment, message delivery is point-to-point, this means, one message is explicitly delivered to one channel. In fact, the deliver_message endpoint payload demands the channel_ref that identifies the channel.

Describe alternatives you've considered
No alternatives considered so far.

Additional context

• An alternate deliver_message endpoint, could request one or more user_refs / channel_refs, and then deliver the message to the active channels.