Enable persistent connections for IbmDb2.

Usage example in global.php:

```php
return array(
    'db' => array(
        'driver' => 'IbmDb2',
        'database' => '*LOCAL',
        'username' => 'user',
        'password' => 'password',
        'persistent' => true,
        'driver_options' => array(
            'i5_naming' => DB2_I5_NAMING_ON,
            'i5_libl' => 'SOMELIB QGPL'
        ),
        'platform_options' => array(
            'quote_identifiers' => false
        )
    ),
    
    'service_manager' => array(
        'factories' => array(
            'Zend\Db\Adapter\Adapter' =>
'Zend\Db\Adapter\AdapterServiceFactory'
        )
    )
);
```

Change-Id: I42ff3d52b2f112cefb828432c93eb409d42b2cad