fix #961

A Pull Request should be associated with an Issue.

We wish to have discussions in Issues. A single issue may be targeted by multiple PRs.
If you're offering a new feature or fixing anything, we'd like to know beforehand in Issues,
and potentially we'll be able to point development in a particular direction.

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

Further notes in https://github.com/github/gh-ost/blob/master/.github/CONTRIBUTING.md
Thank you! We are open to PRs, but please understand if for technical reasons we are unable to accept each and any PR

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.