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.
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.
Add a new API operation to get the overall farm status. This is based on
the jobs and workers, and their status.
The statuses are:
- `active`: Actively working on jobs.
- `idle`: Farm could be active, but has no work to do.
- `waiting`: Work has been queued, but all workers are asleep.
- `asleep`: Farm is idle, and all workers are asleep.
- `inoperative`: Cannot work: no workers, or all are offline/error.
- `starting`: Farm is starting up.
- `unknown`: Unexpected configuration of worker and job statuses.
