docs: @IocBean(name = "mqttClientPersistence")
      public MqttClientPersistence createMqttClientPersistence() {
          switch (conf.get(PROP_PERSISTENCE_TYPE, "memory")) {
          case "file":
              return new MqttDefaultFilePersistence(conf.get(PROP_PERSISTENCE_PATH, System.getProperty("user.dir")));
          default:
              return new MemoryPersistence();
          }
      }