private long[] buildSortedIds(final NavigableSet<EffectiveMeta> set) {
        return set.stream()
                .mapToLong(EffectiveMeta::getId)
                .sorted()
                .toArray();
    }
Cache the hash code as we are pretty much guaranteed to call hashCode in
EffectiveStreamInternPool
stroom-meta/stroom-meta-api/src/main/java/stroom/meta/api/EffectiveMetaSet.java