[2.3] Handle PHP sessions started outside of Symfony

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        | symfony/symfony-docs#2474

This PR brings a way to allow Symfony2 to manage a session started outside of Symfony in such a way that quite explicit. It also introduces more robust detection of previously started sessions under PHP 5.3 and supports real session status detection under PHP 5.4