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.
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