Prepares the appropriate session for the release of Drupal being upgraded.
protected function prepareD8Session() {
  // We need an IP when storing sessions
  // so add a dummy request in the container.
  $request = Request::create('http://example.com/');
  $request->server
    ->set('REMOTE_ADDR', '3.3.3.3');
  $this->container
    ->set('request', $request);
  // Generate and set a D7-compatible session cookie.
  $this
    ->curlInitialize();
  $sid = Crypt::hashBase64(uniqid(mt_rand(), TRUE) . Crypt::randomBytes(55));
  curl_setopt($this->curlHandle, CURLOPT_COOKIE, rawurlencode(session_name()) . '=' . rawurlencode($sid));
  // Force our way into the session of the child site.
  drupal_save_session(TRUE);
  _drupal_session_write($sid, '');
  drupal_save_session(FALSE);
}