private Set<Integer> unionNodeSetWith(final Set<Integer> otherNodes) {
            final Set<Integer> unionNodeSet = new HashSet<>(nodeStates.keySet());
            unionNodeSet.addAll(otherNodes);
            return unionNodeSet;
        }
Maintains a bitset where all non-down nodes have a bit set. All nodes that differ from defaultUp
and defaultDown are stored explicitly in a hash map.
vdslib/src/main/java/com/yahoo/vdslib/state/ClusterState.java