This introduces the concept of 'event listener', which is now used by
the farm status service to respond to events on the event bus.
This makes it possible to reduce the regular poll period from 5 to 30
seconds. That's now only necessary as backup, just in case events are
missed or otherwise things change without the event bus logic noticing.
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.
