function closeStores () {
    that.disconnected = true
    that.incomingStore.close(function () {
      that.outgoingStore.close(cb)
    })
  }
end - close connection

@returns {MqttClient} this - for chaining
@param {Boolean} force - do not wait for all in-flight messages to be acked
@param {Function} cb - called when the client has been closed

@api public