transaction callback added

Added transaction callback shortcut . Related with [this](https://github.com/yiisoft/yii2/issues/3384) issue 
```php
class Post
{
    function getComments()
    {
        // we may use it w/o transaction since it's a single atomic operation
    }

    function getCommentsForDisplay()
    {
        return Yii::$app->db->transaction([$this, 'getComments'], Transaction::READ_UNCOMMITTED);
    }
}
```