Make submissions_failed.chunks_done a not nullable field

In #108 a `NULLABLE` field `chunks_done` was added. It is `NULLABLE` for backwards compatibility reasons only (existing rows in `submissions_failed` don't have a value for this column.

Because rows are flushed from this table after a configurable time we can remove the `NULLABLE` constraint after some time has passed, in a backwards-incompatible API change.

Start by reviewing issue #108 and the schema or migration that added chunks_done to submissions_failed. Check how the table's retention and flushing behavior affect existing rows, then make the column non-nullable once the compatibility period has passed and verify the migration and affected tests succeed.

Written by the indexing model from the issue text.