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
Fix some lint warnings in libphutil

Summary: Lint.

Test Plan: Lint.

Reviewers: chad

Reviewed By: chad

CC: aran

Differential Revision: https://secure.phabricator.com/D6125