Fix MySQL column escaping

Summary:
MySQL doesn't treat \ as escaping character in ``.
This isn't probably SQL injection hole because I've found no calls of this method with user input.
But better safe than sorry.

See also NO_BACKSLASH_ESCAPES.

Test Plan:

SELECT `a\`b`; -- Throws: Syntax error near '`'.
-- Should throw: Unknown column 'a`b'.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran

Differential Revision: https://secure.phabricator.com/D2109