Today in a few places we create a `new CountDown(0)`. This is kind of trappy because it will never return `true` from `#countDown()` or `#fastForward()` which risks never completing whatever action needs completing unless the caller handles the empty case specially.

We should refactor the places that do this so that they all avoid creating an empty `CountDown`, instead just handling the fact that the action is already complete:

- [ ] `TransportIndicesShardStoresAction$AsyncShardStoresInfoFetches`
- [ ] `ClearScrollController`
- [ ] `TransportBroadcastReplicationAction#doExecute`
- [ ] `Realms#usageStats`

Once we've done that, we can forbid the creation of an empty `CountDown`.
x-pack/plugin/security/src/main/java/org/elasticsearch/xpack/security/authc/Realms.java
+ "] realm: ["
                    + (RealmSettings.realmSettingPrefix(misNamedRealm.type()) + misNamedRealm.name())
                    + "]"
            );
        }
    }
    private static Settings ensureOrderSetting(Settings settings, RealmConfig.RealmIdentifier realmIdentifier, int order) {
        String orderSettingKey = RealmSettings.realmSettingPrefix(realmIdentifier) + "order";
        return Settings.builder().put(settings).put(orderSettingKey, order).build();
    }
    private static void checkUniqueOrders(Map<Integer, Set<String>> orderToRealmName) {
        String duplicateOrders = orderToRealmName.entrySet()
            .stream()
            .filter(entry -> entry.getValue().size() > 1)
            .map(entry -> entry.getKey() + ": " + entry.getValue())
            .collect(Collectors.joining("; "));
        if (Strings.hasText(duplicateOrders)) {
            throw new IllegalArgumentException("Found multiple realms configured with the same order: " + duplicateOrders);
        }
    }
    private void ensureUniqueExplicitlyConfiguredRealmNames(Map<String, Set<String>> nameToRealmIdentifier) {
        String duplicateRealms = nameToRealmIdentifier.entrySet()
            .stream()
            .filter(entry -> entry.getValue().size() > 1)
            .map(entry -> entry.getKey() + ": " + entry.getValue())
            .collect(Collectors.joining("; "));
        if (Strings.hasText(duplicateRealms)) {
            throw new IllegalArgumentException("Found multiple realms configured with the same name: " + duplicateRealms + "");
        }
    }