Update the demo protocol plugin to not depend on conversation names

We're starting to phase out conversation names as they don't work well with
modern stuff. So this goes ahead and fixes the not interactions in the demo
protocol.

I also generated the echo message in the send_message_async rather than in the
callback.

Testing Done:
Ran the turtles and send some messages to Echo on the demo protocol.

Reviewed at https://reviews.imfreedom.org/r/3135/