bugzilla: localise all MySQL direct access inside access class. Prepare for the addition of other Bugzilla access methods by localising direct MySQL database access inside an access class. Provide a base access class largely to document the methods required for a class implementing a particular access method. Rename the 'bugzilla_<version>' classes to 'bzmysql_<version>' to emphasise that they are doing access via direct manipulation of a MySQL database.