minor regressions in db_mysql and db_mysqli data wrappers

[AlexKlimenkov]

1. AlexKlimenkov@0247536 MySQLi wrapper must import MySQL wrapper explicitly since it's no longer included in db_common.php
   how to reproduce the issue:

require_once('./connector-php/codebase/scheduler_connector.php');
require_once ("./connector-php/codebase/db_mysqli.php");

--> Fatal error:Class 'MySQLDBDataWrapper' not found in .../db_mysqli.php on line 8

2. AlexKlimenkov@cdf333f MySQL wrapper use classes defined in db_common.php but does not import them. It causes the error If db_mysql.php is included before connector. The error didn't happen in previous versions, other wrappers include db_common explicitly.
   how to reproduce:

// include db_mysql.php before connector.php
require_once ('./connector-php-my/codebase/db_mysql.php'); 
require_once('./connector-php-my/codebase/scheduler_connector.php');

--> Fatal error: Class 'DBDataWrapper' not found in .../db_mysql.php