[HttpFoundation] Allow control over session autostart.
[HttpFoundation] Add emulation mode which emulates behaviour of bare sessions.

This allows bags to be access when a session is not started.
Would be overwritten if session is started which is the same
as $_SESSION which is `null` until initialised. This however
respects the return types since the bag APIs return the
correct types.
[FrameworkBundle] Introduce session control features to configuration.