Enhancement

Before, update schema version and set schema diff in the same transaction. So after schema version grow up, we can get the schema diff at same time.

For now, We don't allow When schema version grow up, schema diff can not be read.

But now, update schema version and set schema diff not in the same transaction.

• TIDB can only make sure the schema diff from lastest schema version - 1 is valuable.
  • If the schema diff from lastest schema version - 1 is empty, Then it means may failed in TIDB, in TiFlash, we can skip it.
  • If the schema diff from lastest schema version - 1 is empty, Then Tiflash should direct apply the schema diff.
• TIDB can't make sure the schema diff with lastest schema version is not empty.

**So we need to allow empty schema diff when schema version increased. **