lightning tidb backend failed when tidb auto commit is not enabled

[glorv]

Please answer these questions before submitting your issue. Thanks!

1. What did you do?
   If possible, provide a recipe for reproducing the error.
   Use lightning tidb backend to load data into tidb who's auto-commit is disabled by default.

2. What did you expect to see?
   Import should succeed.

3. What did you see instead?
   Lightning failed with following error message:

[error="restore table `db`.`table` failed"] [error="Error 8004: transaction too large, len: 300001"]

[SunRunAway]

Why this bug is moderate?

It should be critical because the functionality can not work in this situation.