Fix GH-16314 "Pdo\Mysql object is uninitialized" when opening a persistent connection (#16369)

Implement PDO driver-specific subclasses

RFC: https://wiki.php.net/rfc/pdo_driver_specific_subclasses
Closes GH-12804

Co-Authored-By: Danack <Danack@basereality.com>

ext/pdo_mysql: Optimize tests (#12751)