[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 
From the PHPdoc of the DbalHandler class:

 > This implementation is very similar to Symfony\Component\HttpFoundation\Session\Storage\Handler\PdoSessionHandler but uses a Doctrine connection and thus also works with non-PDO-based drivers like mysqli and OCI8

From the [PR introducing this feature](https://github.com/symfony/symfony/pull/2182#issuecomment-2103612):

 > cleaner code, potentially better support for niche RDBMS, centralized logging and finally afaik DoctrineDBAL has emulation for nested transactions.

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 > The benefits (for me) are:
 > * logging queries
 > * better interoperability with existing build processes (migrations)
 > * better database interoperability
 > * re-using existing connection (I don't have the problem that Stof mentioned above)