coprocessor: let tiflash split range task consistent with tikv

[lzmhhh123]

What problem does this PR solve?

As before, we always set the full range for the region request of TiFlash. But there is some problem when we switch on the region merge. The retry of the merged region may cause the duplicate read. So TiFlash support the range in coprocessor and TiDB make the range split consistent with TiKV.

What is changed and how it works?

Make the range split of TiFlash region request consistent with TiKV.

Check List

Tests

• Unit test
• Integration test

[windtalker]

Does the previous changes in https://github.com/pingcap/tidb/blob/master/planner/core/find_best_task.go#L1085
need to be reverted?

[lzmhhh123]

Does the previous changes in /planner/core/find_best_task.go@master#L1085
need to be reverted?

I think we'd better not. The process of CBO would be affected.

[windtalker]

Does the previous changes in /planner/core/find_best_task.go@master#L1085
need to be reverted?

I think we'd better not. The process of CBO would be affected.

Ok, but better to revert it in the future. And the changes in https://github.com/pingcap/tidb/blob/master/planner/core/explain.go#L153
should be reverted.

[lzmhhh123]

Does the previous changes in /planner/core/find_best_task.go@master#L1085
need to be reverted?

I think we'd better not. The process of CBO would be affected.

Ok, but better to revert it in the future. And the changes in /planner/core/explain.go@master#L153
should be reverted.

OK.

[windtalker]

LGTM

[crazycs520]

LGTM

[lzmhhh123]

/merge

[sre-bot]

/run-all-tests

[sre-bot]

@lzmhhh123 merge failed.

[lzmhhh123]

/run-unit-test