Econimizing fetching, getting exhaustive playlist data only if necessary.

Determine whether we need to do an exhaustive fetch
(.get_all_user_playlist_contents()) according to .get_all_playlists()
lastModifiedTimestamp, and proceed accordingly.  (lastModifiedTimestamp
apparently wasn't useful, because it wasn't kept up-to-date, until
recently.)

Rectify reporting info.

Include a way - migrate_version() - to migrate the collected stats to a
new instance, for development increments that can exercise the
progressive fetching.