CDC exit unexpected and cannot recover automatically

[Tammyxia]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do? If possible, provide a recipe for reproducing the error.

• Create 2x changefeed with different target but the same source database.
• "sysbench oltp_common prepare ... --threads=512 --tables=1000 --table-size=0" to upstream
  Check CDC status.

2. What did you expect to see?

• sync task works fine.

3. What did you see instead?

• CDC exit . "tiup cdc cli capture list --pd=http://172.16.4.96:2379" has no any cdc process.
  Waiting for an hour, CDC still cannot start automatically.

• Some cdc log:

4. Versions of the cluster

   • Upstream TiDB cluster version (execute SELECT tidb_version(); in a MySQL client):

     Edition: Community
     

Git Commit Hash: d5e0ed0aaed72d2f2dfe24e9deec31cb6cb5fdf0
Git Branch: master
UTC Build Time: 2021-05-24 14:39:20
GoVersion: go1.13
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false |
```

- TiCDC version (execute `cdc version`):

    ```
   Release Version: v5.0.0-master-dirty

Git Commit Hash: ca64c78
Git Branch: master
UTC Build Time: 2021-05-31 05:29:05

    ```