MQTT Driver: Add last seen / last updated timestamp tag to MQTT Device

[ash_aqx]

The current fields relate to whether the device is successfully subscribed to a topic. It would be valuable if there was a tag that referenced the latest timestamp received by the Device.

Schema A requires a "time" field it would be great if this was part of the Device object somewhere

Use Case:

I have a large number of MQTT devices. I need a very basic health check of, has my device ever received data? When was it last seen?

Depending on what is pushed to the topic the points updated in the Device could be all, one, or anything in between. 

[sbeadle]

Thank you.