There's at least one example of stored routes and advertised routes
getting out of sync. I don't know how they got there yet, but this would
backfill missing advertised routes on startup from stored routes.
Also add logging in LocalBackend.AdvertiseRoute to record when new
routes actually get put into prefs.
Updates #14606
Signed-off-by: Andrew Lytvynov <awly@tailscale.com>