Escape MySQL column names

[andres-lowrie]

This change wraps all the column names with backticks so that MySQL won't complain when a table has columns that are also reserved keywords in MySQL.

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This change is 

[andres-lowrie]

I was migrating a database that uses MySQL reserved keywords for column names thus the migrate tool was showing the following MySQL error when trying to reference said column in a query:

while writing data file: while dumping table food: Error 1064: 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 'groups,id,in_fndds,long_description,manufacturers,refuse_description,refuse_perc' at line 1

The problem was with the column name groups, this I figured surrounding the column name with backticks should allow users to easier migrate a larger variety of MySQL tables.

[andres-lowrie]

Reviewed 1 of 1 files at r2. Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @andres-lowrie and @manishrjain)

dgraph/cmd/migrate/dump.go, line 95 at r2 (raw file):

Quoted 4 lines of code… using the same logic twice. This could be wrapped in a method.

I put in a function instead of method out of habit and figured that the wrapping logic was generic enough, idk

If this should be method I'm glad to change it

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[mangalaman93]

Feel free to re-open this PR against main, closing it for now. Thank you for the PR.