CREATE USER/GRANT PRIVILEGE should begin a new transaction

[tiancaiamao]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?
   If possible, provide a recipe for reproducing the error.

mysql> begin;                                                                                                                                                                                         Query OK, 0 rows affected (0.00 sec)

mysql> create user 'xxx'@'%';                                                                                                                                                                       
Query OK, 1 row affected (0.01 sec)

mysql> grant all privileges on *.* to 'xxx'@'%';                                                                                                                                                    
ERROR 1062 (23000): Duplicate entry '%-xxx' for key 'PRIMARY'

2. What did you expect to see?

Success

3. What did you see instead?

Fail

4. What version of TiDB are you using (tidb-server -V or run select tidb_version(); on TiDB)?

2.1, 3.0, master ... all of them

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Just like DDL, create user grant xxx should begin a new transaction