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.
SocketIO has 'rooms' and 'event types'. The 'event type' is set via
reflection of the OpenAPI type of the event payload. This has to be set
up in a mapping, though, and if that mapping is incomplete, an error will
now be logged.
Fix SocketIO subscriptions so that the client also subscribes to
job-specific last-rendered images whenever subscribing to job-specific
events. These are sent to another event topic, and thus need some extra
care. Before the introduction of the generic event bus, both message types
were sent to the same topic, but that's not supported by MQTT, and so things
had to change.
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.
