…n the MemcachedSessionHandler (Alessandro Loffredo)

This PR was merged into the 3.4 branch.

Discussion
----------

[Fix][3.4][HttpFoundation] Fix the updating of timestamp in the MemcachedSessionHandler

| Q             | A
| ------------- | ---
| Branch?       | 3.4
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT

Conditions: Symfony 3.4, PHP7 and sessions handled over memcache.

Apparently `memcached::touch()` returns `false` on a subsequent call with the same parameters. Since `updateTimestamp` is used in `AbstractSessionHandler::write()`

```
public function write($sessionId, $data)
    {
        if (\PHP_VERSION_ID < 70000 && $this->prefetchData) {
            $readData = $this->prefetchData;
            $this->prefetchData = null;

            if ($readData === $data) {
                return $this->updateTimestamp($sessionId, $data);
            }
        }
...
```

the result is that `write()` will return `false` on **any subsequent request within the same second** causing the following error:

```
HP Fatal error:  Uncaught Symfony\Component\Debug\Exception\ContextErrorException: Warning: session_write_close(): Failed to write session data (user). Please verify that the current setting of session.save_path is correct (/var/lib/php/sessions) in Unknown:0
Stack trace:
#0 [internal function]: Symfony\Component\Debug\ErrorHandler->handleError(2, 'session_write_c...', 'Unknown', 0, NULL)
#1 [internal function]: session_write_close()
#2 {main}
  thrown in Unknown on line 0
```

Can be reproduced on `symfony/skeleton:3.4` adding the following code to `public/index.php` and performing two consecutive requests:
```
$session = $kernel->getContainer()->get('session');
$session->set("foo", "bar");
```

Commits
-------

d007469 fix the updating of timestamp in the MemcachedSessionHandler