config: setter/getter for Notifier and Logger into its own struct. (#3721)

This is an attempt cleanup code and keep the top level config
functions simpler and easy to understand where as move the
notifier related code and logger setter/getter methods as part
of their own struct.

Locks are now held properly not globally by configMutex, but
instead as private variables.

Final fix for #3700

cmd/notify-webhook.go

@@ -52,7 +52,7 @@ func lookupEndpoint(u *url.URL) error {
 
 // Initializes new webhook logrus notifier.
 func newWebhookNotify(accountID string) (*logrus.Logger, error) {
-	rNotify := serverConfig.GetWebhookNotifyByID(accountID)
+	rNotify := serverConfig.Notify.GetWebhookByID(accountID)
 
 	if rNotify.Endpoint == "" {
 		return nil, errInvalidArgument