Display the last path update date and dead paths indicator

[tonybentley]

Currently the signalk model provides the timestamp for each value and persists the value regardless of the last updated date/time. If the instrument fails to continue updating, the model is not cleared so the user does not know if the data is from 2 seconds ago, or 2 days ago.

Provide the following format in the instrument display:

Showing {dd} days, {hh} hours, {mm} minutes, {ss} seconds ago
If today and last updated {dd} was today then omit, {hh} is current hour then omit, etc

[cmotelet]

As since version 1.20.0 of the server it is possible to add a metadata timeout per path we will be able in only one operation to have the date of the last value and an alarm in case of no update.