code: public void clearAllSubscriptions() {
          SubscriptionTreeCollector subsCollector = new SubscriptionTreeCollector();
          bfsVisit(subscriptions, subsCollector);
          
          List<Subscription> allSubscriptions = subsCollector.getResult();
          for (Subscription subscription : allSubscriptions) {
              removeSubscription(subscription.getTopic(), subscription.getClientId());
          }
      }
docs: TODO implement testing