public boolean isEmpty() {
        return delayQueue.isEmpty();
    }
Register the refreshable so that its refreshing gets managed. Can be called many times
for the same {@link Refreshable}, e.g. to notify the {@link RefreshManager} that it was
refreshed externally.
stroom-security/stroom-security-common-impl/src/main/java/stroom/security/common/impl/RefreshManager.java