Starts the session.
@api
boolean True if started.
\RuntimeException If something goes wrong starting the session.
Overrides SessionStorageInterface::start
public function start() {
  if ($this->started && !$this->closed) {
    return true;
  }
  if (empty($this->id)) {
    $this->id = $this
      ->generateId();
  }
  $this
    ->loadSession();
  return true;
}