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
if (activeRealms != null) {
            activeRealms.stream()
                .filter(r -> licensedRealms.contains(r) == false)
                .forEach(realm -> { handleDisabledRealmDueToLicenseChange(realm, licenseStateSnapshot); });
        }
        activeRealms = licensedRealms;
    }
    
    protected void handleDisabledRealmDueToLicenseChange(Realm realm, XPackLicenseState licenseStateSnapshot) {
        final LicensedFeature.Persistent feature = getLicensedFeatureForRealm(realm.type());
        assert feature != null
            : "Realm ["
                + realm
                + "] with no licensed feature became inactive due to change to license mode ["
                + licenseStateSnapshot.getOperationMode()
                + "]";
        feature.stopTracking(licenseStateSnapshot, realm.name());
        logger.warn(
            "The [{}.{}] realm has been automatically disabled due to a change in license [{}]",
            realm.type(),
            realm.name(),
            licenseStateSnapshot.statusDescription()
        );
    }
    @Override
    public Iterator<Realm> iterator() {
        return getActiveRealms().iterator();
    }
    /**
     * Returns a list of realms that are configured, but are not permitted under the current license.