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.

首先检查 issue #108，以及向 submissions_failed 添加 chunks_done 的 schema 或 migration。检查表的保留和 flush 行为如何影响现有行，然后在兼容期结束后将该列设为 non-nullable，并验证 migration 和受影响的测试是否成功。

由索引模型根据 Issue 内容生成。