消息的发送时间戳（以秒为单位）。
Timestamps can be used to implement interpolation or extrapolation of continous streams of packets
The timestamp is passed as a double to avoid overflow when a game is running for a long time.
Internally timestamps are sent as 32 bit integers with millisecond accuracy to save bandwidth.
Timestamps are automatically adjusted to be relative to Network.time.
Thus Network.time - messageInfo.timeStamp is the time the packet spent in transit.