Support for flush status

[jackysp]

Please answer these questions before submitting your issue. Thanks!

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

2. What did you expect to see?
   no error

3. What did you see instead?

mysql> flush status;
ERROR 1105 (HY000): line 1 column 12 near "" (total length 12)

4. What version of TiDB are you using (tidb-server -V or run select tidb_version(); on TiDB)?
   0d18ac2

[jackysp]

Please add the syntax support for flush status first.