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