Requst to allow the server to add a timestamp

We have received a comment on the MQTTv5.0 spec from an external location and it is being echoed here so others can see the comment.  Issue MQTT-504 https://issues.oasis-open.org/browse/MQTT-504 (No way to tell when a retained message was sent) has been opened to document this comment.

The original comment is:
https://twitter.com/MichMich/status/903283262415568898

Allow the server to set a property with the received timestamp on the retained message.  

Ken Borgendale  --