Fixed session autostarting issue

Using this bundle with a reverse cache solution (like `Varnish`) is currently impossible, because when rendering, for example, the `async.html.twig` template, it will automatically initialize the session, whether it's used or not. This pull request solves this problem by just checking first if the sessions has started or not. It does not broke the normal behaviour adding AdWords conversion, custom PageViews, etc. because they will automatically initialize the session, likely from the controller.