lib/model, lib/config: Support "live" device removal, folder unsharing and folder configuration changes

Furthermore:
1. Cleans configs received, migrates them as we receive them.
2. Clears indexes of devices we no longer share the folder with

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3478

cmd/syncthing/main.go

@@ -671,7 +671,6 @@ func syncthingMain(runtimeOptions RuntimeOptions) {
 	}
 
 	m := model.NewModel(cfg, myID, myDeviceName(cfg), "syncthing", Version, ldb, protectedFiles)
-	cfg.Subscribe(m)
 
 	if t := os.Getenv("STDEADLOCKTIMEOUT"); len(t) > 0 {
 		it, err := strconv.Atoi(t)