Start the session. Invoke in your @gen.coroutine wrapped prepare
        method like::

            result = yield gen.Task(self.start_session)

        :rtype: bool

def start_session(self):
        """Start the session. Invoke in your @gen.coroutine wrapped prepare
        method like::

            result = yield gen.Task(self.start_session)

        :rtype: bool

        """
        self.session = self._session_start()
        result = yield gen.Task(self.session.fetch)
        self._set_session_cookie()
        if not self.session.get('ip_address'):
            self.session.ip_address = self.request.remote_ip
        self._last_values()
        raise gen.Return(result)