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
+ kerberosRealmNames.toString()
                            + " configured of type ["
                            + identifier.getType()
                            + "], ["
                            + identifier.getType()
                            + "] can only have one such realm "
                            + "configured"
                    );
                }
            }
            realmConfigs.add(config);
        }
        return realmConfigs;
    }
    private static Set<String> findDisabledBasicRealmTypes(List<RealmConfig> realmConfigs) {
        return realmConfigs.stream()
            .filter(rc -> InternalRealms.isBuiltinRealm(rc.type()))
            .filter(rc -> false == rc.enabled())
            .map(RealmConfig::type)
            .collect(Collectors.toUnmodifiableSet());
    }
    private static void logDeprecationForReservedPrefixedRealmNames(List<RealmConfig.RealmIdentifier> realmIdentifiers) {
        if (false == realmIdentifiers.isEmpty()) {
            deprecationLogger.warn(
                DeprecationCategory.SECURITY,
                "realm_name_with_reserved_prefix",
                "Found realm "
                    + (realmIdentifiers.size() == 1 ? "name" : "names")
                    + " with reserved prefix [{}]: [{}]. "
                    + "In a future major release, node will fail to start if any realm names start with reserved prefix.",