Feature: Persist last known value, source, timestamp, and metadata for each path upon restarting

[tonybentley]

Scenario:

As a user I want to know my last known value for each path in Signalk server upon restarting the signalk-server application.

Given there are paths that are only updated on a 1 times per day basis

When I restart signalk-server the paths are not persisted

Acceptance criteria:

When I restart signalk-server I should see the last known value for each path in the data explorer
And I should see the timestamp, metadata, and source from the last known update