STARTED jobs with a non empty "finished_at" field
Some jobs cannot be finished correctly and get stuck in the STARTED status (see https://github.com/huggingface/datasets-preview-backend/blob/9e360f9de0df91be28001587964c1af25f82d051/src/datasets_preview_backend/io/queue.py#L253)

```
36|worker-splits-A  | DEBUG: 2022-02-25 10:32:44,579 - datasets_preview_backend.io.cache - split 'validation' from dataset 'katanaml/cord' in config 'cord' had error, cache updated
36|worker-splits-A  | WARNING: 2022-02-25 10:32:44,593 - datasets_preview_backend.io.queue - started job SplitJob[katanaml/cord, cord, validation] has a non-empty finished_at field. Aborting.
```

<img width="387" alt="Capture d’écran 2022-02-25 à 11 33 40" src="https://user-images.githubusercontent.com/1676121/155700317-b6f6813f-9260-44b6-878c-20debbe5444b.png">

STARTED jobs with a non empty "finished_at" field: Some jobs cannot be finished correctly and get stuck in the STARTED status (see https://github.com/huggingface/datasets-preview-backend/blob/9e360f9de0df91be28001587964c1af25f82d051/src/datasets_preview_backend/io/queue.py#L253)

```
36|worker-splits-A  | DEBUG: 2022-02-25 10:32:44,579 - datasets_preview_backend.io.cache - split 'validation' from dataset 'katanaml/cord' in config 'cord' had error, cache updated
36|worker-splits-A  | WARNING: 2022-02-25 10:32:44,593 - datasets_preview_backend.io.queue - started job SplitJob[katanaml/cord, cord, validation] has a non-empty finished_at field. Aborting.
```

<img width="387" alt="Capture d’écran 2022-02-25 à 11 33 40" src="https://user-images.githubusercontent.com/1676121/155700317-b6f6813f-9260-44b6-878c-20debbe5444b.png">
