{HttpFoundation] [Session] fixed session compatibility with memcached/redis session storage

Per https://bugs.php.net/bug.php?id=61470, and in fixing #7380, the following error occurs when using a Memcache or Redis session store w/ Symfony security: "Authentication exception occurred; redirecting to authentication entry point (A Token was not found in the SecurityContext.)".  This patch applies the first fix only if the session store is "files"