[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 
@WouterJ I don't understand your statement ``re-using existing connection (I don't have the problem that Stof mentioned above)``. If you use the same connection for your session storage and for your application logic, you will suffer from the issue when a transaction is rollbacked. there is no way you cannot be affected