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
private final Settings settings;
    private final Environment env;
    private final Map<String, Realm.Factory> factories;
    private final XPackLicenseState licenseState;
    private final ThreadContext threadContext;
    private final ReservedRealm reservedRealm;
    
    private final List<Realm> allConfiguredRealms;
    private final Map<String, DomainConfig> domainNameToConfig;
    
    private final Map<RealmConfig.RealmIdentifier, Authentication.RealmRef> realmRefs;
    
    private volatile List<Realm> activeRealms;
    public Realms(
        Settings settings,
        Environment env,
        Map<String, Realm.Factory> factories,
        XPackLicenseState licenseState,
        ThreadContext threadContext,
        ReservedRealm reservedRealm
    ) throws Exception {
        this.settings = settings;
        this.env = env;
        this.factories = factories;
        this.licenseState = licenseState;
        this.threadContext = threadContext;
        this.reservedRealm = reservedRealm;
        assert XPackSettings.SECURITY_ENABLED.get(settings) : "security must be enabled";
        assert factories.get(ReservedRealm.TYPE) == null;
        final Map<String, DomainConfig> realmToDomainConfig = RealmSettings.computeRealmNameToDomainConfigAssociation(settings);