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
domainNameToConfig = realmToDomainConfig.values()
            .stream()
            .distinct()
            .collect(Collectors.toMap(DomainConfig::name, Function.identity()));
        final List<RealmConfig> realmConfigs = buildRealmConfigs();
        
        final List<Realm> initialRealms = initRealms(realmConfigs);
        realmRefs = calculateRealmRefs(realmConfigs, realmToDomainConfig);
        initialRealms.forEach(realm -> realm.initRealmRef(realmRefs));
        this.allConfiguredRealms = initialRealms;
        this.allConfiguredRealms.forEach(r -> r.initialize(this.allConfiguredRealms, licenseState));
        assert this.allConfiguredRealms.get(0) == reservedRealm : "the first realm must be reserved realm";
        this.activeRealms = calculateLicensedRealms(licenseState.copyCurrentLicenseState());
        licenseState.addListener(this::recomputeActiveRealms);
    }
    private Map<RealmConfig.RealmIdentifier, Authentication.RealmRef> calculateRealmRefs(
        Collection<RealmConfig> realmConfigs,
        Map<String, DomainConfig> domainForRealm
    ) {
        final String nodeName = Node.NODE_NAME_SETTING.get(settings);
        final Map<RealmConfig.RealmIdentifier, Authentication.RealmRef> realmRefs = new HashMap<>();
        assert realmConfigs.stream().noneMatch(rc -> rc.name().equals(reservedRealm.name()) && rc.type().equals(reservedRealm.type()))
            : "reserved realm cannot be configured";
        realmRefs.put(
            new RealmConfig.RealmIdentifier(reservedRealm.type(), reservedRealm.name()),
            new Authentication.RealmRef(reservedRealm.name(), reservedRealm.type(), nodeName, null)
        );
        for (RealmConfig realmConfig : realmConfigs) {
            final RealmConfig.RealmIdentifier realmIdentifier = new RealmConfig.RealmIdentifier(realmConfig.type(), realmConfig.name());
            final DomainConfig domainConfig = domainForRealm.get(realmConfig.name());