RefreshKeys approach not workable

[bbockelm]

This is a deferred work item from #1748.

The RefreshKeys API is flawed. It doesn't work reliably as it assumes there are no updates missed (updates that could occur when the host is offline or if there's other RefreshKeys calls added in the future).

We should expand things so we;

• Periodically register the keys regardless (can be tens of minutes between periods).
• Always register the keys on startup.
• Either prevent missing updates or detect missed updates. Two possible approaches:
  • Allow a callback to be registered by launcher_utils in config. This requires config to have an active goroutine running which I don't particularly like.
  • Have the RefreshKeys become something that computes a stable tag (version number, checksum) and returns it. Then the loop can detect if the tag has changed since the last time.