integration: clean up tidb dependency

[disksing]

Due to historical reason, integration_test now imports pingcap/tidb. The dependency update issues it carries with it are often painful during the development process.

The main reason why tidb must be imported at the moment is that some transactional advanced interfaces are only implemented in tidb/unistore and not in mockstore. So we plan to replace the tests using unistore with mockstore in turn and make them pass the tests.

Subtasks:

• [ ] #441
• [x] #442
• [ ] #443
• [ ] #444