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