R1:e69ba98e2006

Prepare for MySQLi support

Summary: This separates common MySQL stuff (identifiers and comments escaping, error codes, connection retries) from PHP extension specific stuff (connect, query, fetch, errors, escape string).

Test Plan:
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Use `AphrontMySQLiDatabaseConnection` in `PhabricatorLiskDAO`, load homepage, edit task, save task.

Reviewers: epriestley

Reviewed By: epriestley

CC: nh, aran

Differential Revision: https://secure.phabricator.com/D2113