Add a simple interface to abstract database escaping from aphront

Summary: Currently the qsprintf require an AphrontDatabaseConnection to escape strings which prevents others from using qsprintf without using aphront.  Instead, this creates an interface that the AphrontMySQLDatabaseConnection implements that then allows any implementer to use qsprintf.

Test Plan: Not actually sure how to test this.  @epriestley: Can you suggest some proper ways to test this besides "use phabricator"?

Reviewers: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D6123