[WIP]  Added DBAL session storaged based on PDO Session storage page

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | 2.1
| Fixed tickets | #881

I added a quick reference to https://github.com/symfony/symfony/blob/2.1/src/Symfony/Bridge/Doctrine/HttpFoundation/DbalSessionHandler.php basing it almost completely on the PDO Session handler

I'm merging it into this branch, even though it's not mantained anymore, because this is where this option became available to use for session storage 
I like this note! At the end, let's link here: http://symfony.com/doc/current/cookbook/doctrine/multiple_entity_managers.html