Cannot run batch prediction on selected checkpoint

How to replicate:

- Create project
- Run live prediction on a small area
- Retrain a couple times
- Create large new AOI
- Run batch prediction, once completed, its checkpoint will be not the result of the last retrain (passed as parameter to POST /instance), but a new one with parent equal to `null`

Some captures I did when replicating this bug:

- Batch prediction request, `checkpoint_id` equal to 158:

- Batch prediction status during processing:

- Batch prediction response payload when completed, the `checkpoint_id` is 159:

- Newly created checkpoint doesn't have a parent, which make the interface behave as it is the base model:

@LanesGood this issue should be probably the cause of https://github.com/developmentseed/pearl-frontend/issues/126.

cc @ingalls @geohacker