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.
