[Kernel] ensure session is saved before sending response

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #6417, #7885
| License       | MIT
| Doc PR        | n/a

Saves the session, in case it is still open, before sending the response.

This ensures several things in case the developer did not save the session explicitly:

- If a session save handler without locking is used, it ensures the data is available
on the next request, e.g. after a redirect. PHPs auto-save at script end via
session_register_shutdown is executed after fastcgi_finish_request. So in this case
the data could be missing the next request because it might not be saved the moment
the new request is processed.

- A locking save handler (e.g. the native 'files') circumvents concurrency problems like
the one above. By saving the session before long-running things in the terminate event,
we ensure the session is not blocked longer than needed.

- When regenerating the session ID no locking is involved in PHPs session design. See
https://bugs.php.net/bug.php?id=61470 for a discussion. So in this case, the session must
be saved anyway before sending the headers with the new session ID. Otherwise session
data could get lost again for concurrent requests with the new ID. One result could be
that you get logged out after just logging in.

This listener should be executed as one of the last listeners, so that previous listeners
can still operate on the open session. This prevents the overhead of restarting it.
Listeners after closing the session can still work with the session as usual because
 Symfonys session implementation starts the session on demand. So writing to it after
it is saved will just restart it.