Use Symfony2's session system.

Our Session component has gotten old and rusty, I propose we replace it with Symfony2's session system.

The API is very similar. We'll keep providing all storage engines we used to provide that Symfony2 does not provide, especially one for our database system, but also Xcache and APC.

What's missing is the token code. We'll have to provide this code later on. However we need a redesign here anyway, our old session code was tightly coupled to the application which is not acceptable anymore.