fix #961

[Fanduzi]

fix github#961

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Related issue: https://github.com/github/gh-ost/issues/0123456789

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Description

https://dev.mysql.com/doc/refman/8.0/en/keywords.html
ROWS (R); became reserved in 8.0.2

if we use -exact-rowcount option, gh-ost will execute query

select /* gh-ost */ count(*) as rows from `db`.`t1`;

which will cause

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 'rows from `db`.`t1`' at line 1

Please use ```

select /* gh-ost */ count(*) as `rows` from `db`.`t1`;

In case this PR introduced Go code changes:

• contributed code is using same conventions as original code
• script/cibuild returns with no formatting errors, build errors or unit test errors.