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-kafka.go

@@ -75,7 +75,7 @@ func dialKafka(kn kafkaNotify) (kafkaConn, error) {
 }
 
 func newKafkaNotify(accountID string) (*logrus.Logger, error) {
-	kafkaNotifyCfg := serverConfig.GetKafkaNotifyByID(accountID)
+	kafkaNotifyCfg := serverConfig.Notify.GetKafkaByID(accountID)
 
 	// Try connecting to the configured Kafka broker(s).
 	kc, err := dialKafka(kafkaNotifyCfg)