Rename base MySQL database connection class
Summary: Ref T5655. Rename AphrontMySQLDatabaseConnectionBase to AphrontBaseMySQLDatabaseConnection for consistency.
Test Plan: N/A
Reviewers: Blessed Reviewers, epriestley
Reviewed By: Blessed Reviewers, epriestley
Subscribers: epriestley, Korvin
Maniphest Tasks: T5655
Differential Revision: https://secure.phabricator.com/D10148