Test server tether-io.dev should run Eclipse MQTT

[anselanza]

RabbitMQ is not compliant with a lot of MQTT features, including:

• retained messages
• invalid wildcard subscriptions

[anselanza]

Since it's now so easy to get MQTT up and running using a single Docker command.... do we even need this server?

[rckok]

I don't think we do. I've not used it for ages anyway. I'm fine with deprecating it, including everything we have on there (i.e. the older tether packages)

[anselanza]

Let's delete!