Leveled Architecture: main --> flow --> transactional_operation --> dbctrl

Principles:
- SQL statement should not be out of 'dbctrl'.
- 'flow' should not have specified execution detail.
- All operations in 'transactional_operation' should be in a Transaction.
- Single direction calling.

https://pingcap.feishu.cn/wiki/MaR1wgfM4iZVa5k9af6c150WnXe# tidb-compare
# tidb-compare