Error when `sql_small_result`/`sql_big_result+distinct`/`distinctrow`

[actiontech-bot]

Raised by: @FlyingMao
Steps:

1. Query: select sql_big_result distinct pad from sbtest1;
   Return:

You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'AS pad
FROM sbtest1
LIMIT 100' at line 1

2. Query: select sql_small_result distinct pad from sbtest1 order by pad;
   Return: io.mycat.plan.common.exception.MySQLOutPutException:column distinct not found

[actiontech-bot]

By @yanhuqing666

1. distinctrow case should be fixed.
2. Parser support only select distinct sql_small_result.
   When parsing select sql_small_result distinct column, distinct will be treated as column name, and column as alias name. The SQL will be treated as select sql_small_result distinct as column, which cause 1064 error.
   Solution: Usage should follow MySQL documentation.