{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"

{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"
merged branch klmatom/ticket_8460 (PR #8776)

This PR was submitted for the master branch but it was merged into the 2.2 branch instead (closes #8776).

Discussion
----------

{HttpFoundation] [Session] fixed session compatibility with memcached/re...

...dis session storage

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | no (no new failng/errors introduced)
| Fixed tickets | 7380
| License       | MIT

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"

Commits
-------

eb8d8eb {HttpFoundation] [Session] fixed session compatibility with memcached/redis session storage
fixed CS