AMQP: FBP discovery messages should be sent on fanout exchange

[jonnor]

That way the messages will not crop up in an unused queue when there is no coordinator,
and there can be multiple consumers of the messages instead of just one.

Requires changes in all the participant libraries, the coordinator and to the documentation.

• msgflo-nodejs
• msgflo-python
• noflo-runtime-msgflo
• msgflo-cpp
• msgflo-rust

[jonnor]

This is a breaking protocol change. So far, as far as we know, only the projects under the 'MsgFlo' github organization uses this.

[bergie]

If the discovery thing becomes and exchange, it might also make sense to send the "Hello World I'm Here" message periodically and not only at start-up. This way coordinator can catch participants even if they started before coordinator joined the game

[jonnor]

I also think that when we do this breaking change, we should rename this queue to something msgflo instead of fbp. Because what travels along are not FBP runtime protocol messages, but msgflo discovery messages. There are many concepts which are closely related or reused, so that a whole service of MsgFlo participant can be seen as one FBP runtime - but I think this distinction will help understand the differences.

[jonnor]

@bergie see #30 for periodic sending of discovery messages

[jonnor]

More important now that periodic sending is (starting to be) implemented, #30