Don't lower case keywords

[systay]

Before this change, non-reserved keywords would get their case changed when parsed.

SELECT Status FROM Table

was parsed as:

SELECT `status` FROM Table

[systay]

This fixes #5821

[harshit-gangal]

LGTM

@sougou to confirm and merge.

[saifalharthi]

Looks good

[morgo]

@systay I hate to suggest it - but you may need to add an option to temporarily revert the previous behavior. Apps that were built for Vitess may refer to columns in result sets in a case-sensitive way (they typically either do this or refer by ordinal position).

We can say the option is deprecated on-launch and will be removed in a future version. The default behavior should be as in this PR.

[harshit-gangal]

@morgo The issue of lower casing was only with non reserved vitess keywords.
i.e for Other column names the case is maintained as-is.
If any application would have been case sensitive, then they either are using only lower case table column names or they are using case insensitive column name matching to retrieve the row information.

[morgo]

@harshit-gangal Sorry for my misunderstanding, that sounds better. Let's merge, and document as such.