Send an event to the event bus whenever the farm status changes. The event
contains a farm status report (like `{status: "active"}`), and is sent to
the `/status` topic.
Note that at this moment the status is only polled every X seconds, and
thus may lag behind other events.
Send events on Manager startup & shutdown. To make this possible, events
sent to MQTT are now queued up until an MQTT server can be reached.
Otherwise the startup event would be sent before the MQTT connection was
established.
Introduce an "event bus"-like system. It's more like a fan-out
broadcaster for certain events. Instead of directly sending events to
SocketIO, they are now sent to the broker, which in turn sends it to any
registered "forwarder". Currently there is ony one forwarder, for
SocketIO.
This opens the door for a proper MQTT client that sends the same events
to an MQTT server.
