code: def start_session(self):
          
          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)
docs: Start the session. Invoke in your @gen.coroutine wrapped prepare
          method like::
  
              result = yield gen.Task(self.start_session)
  
          :rtype: bool