Feature request: Support CHECKSUM TABLE command

When importing data into TiDB we want to be sure the data is correct. A convenient command in MySQL is CHECKSUM TABLE and this command is not currently supported by TiDB.

Adding it would make it easy to confirm if the data loaded in from an external source matches or not.

Example I did something like:

$ ssh mysqlhost sudo mysqldump test SomeTable | mysql -h tidbhost -P 4000 -u root -D test

MySQL 5.6:

root@mysqlhost [test]> checksum table SomeTable;
+-----------+------------+
| Table     | Checksum   |
+-----------+------------+
| SomeTable | 2867272629 |
+-----------+------------+
1 row in set (0.01 sec)

TiDB:

root@tidbhost [test]> checksum table SomeTable;
ERROR 1105 (HY000): line 0 column 8 near " table SomeTable"
root@tidbhost [test]> 

So providing some sort of CHECKSUM table command would be great.