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

[tkurki]

I think this makes sense and have wished for it a few times myself.

Probably requires #1812 so that all data is removed eventually, otherwise some may linger forever. And a way to explicitly flush the latest values, from UI and from a start command option.

Dumping all latest deltas every once in a while to a json file is the simplest way to do this, but I wonder if an sqlite database with columns

• context
• path
• value
• $source
• timestamp
  would be a better solution. That could be updated for every received delta.

In principle we should also capture the state of async actions, but I don't think that is feasible or worthwhile.

[KEGustafsson]

Old issue #1308, would be good to link here too. Plugins to have possibility to save their data/states/etc.. if server is restarted.