bug symfony#26403 fix the handling of timestamp in the MongoDBSessionHandler (hjanuschka)

This PR was submitted for the master branch but it was squashed and merged into the 4.0 branch instead (closes symfony#26403).

Discussion
----------

fix the handling of timestamp in the MongoDBSessionHandler

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

in the process of upgrading from 3.4 to 4.0 we stumbled upon a issue with mongo session handler.

```
[05-Mar-2018 11:12:57 Europe/Vienna] PHP Fatal error:  Uncaught Error: Call to undefined method Symfony\Component\HttpFoundation\Session\Storage\Handler\MongoDbSessionHandler::createDateTime() in /opt/APP/vendor/symfony/http-foundation/Session/Storage/Handler/MongoDbSessionHandler.php:144
Stack trace:
#0 [internal function]: Symfony\Component\HttpFoundation\Session\Storage\Handler\MongoDbSessionHandler->updateTimestamp('96d983b59f8aef8...', 'user_obj|O:9:"k...')
#1 [internal function]: session_write_close()
#2 {main}
  thrown in /opt/APP/vendor/symfony/http-foundation/Session/Storage/Handler/MongoDbSessionHandler.php on line 144
```

this PR re-add's the method, that somehow got removed in 4.0 branch, whereas the interface forces to have the implemantation.

Commits
-------

97d9ea8 fix the handling of timestamp in the MongoDBSessionHandler

Author: stof

src/Symfony/Component/HttpFoundation/Session/Storage/Handler/MongoDbSessionHandler.php

@@ -141,7 +141,7 @@ protected function doWrite($sessionId, $data)
      */
     public function updateTimestamp($sessionId, $data)
     {
-        $expiry = $this->createDateTime(time() + (int) ini_get('session.gc_maxlifetime'));
+        $expiry = new \MongoDB\BSON\UTCDateTime((time() + (int) ini_get('session.gc_maxlifetime')) * 1000);
 
         if ($this->mongo instanceof \MongoDB\Client) {
             $methodName = 'updateOne';
@@ -154,7 +154,7 @@ public function updateTimestamp($sessionId, $data)
         $this->getCollection()->$methodName(
             array($this->options['id_field'] => $sessionId),
             array('$set' => array(
-                $this->options['time_field'] => $this->createDateTime(),
+                $this->options['time_field'] => new \MongoDB\BSON\UTCDateTime(),
                 $this->options['expiry_field'] => $expiry,
             )),
             $options