private void setNodeStateInternal(int index, NodeState ns) {
            nodeStates.remove(index);
            if (ns.getState() == State.DOWN) {
                upNodes.clear(index);
                if ( ! equalsWithDescription(defaultDown(), ns)) {
                    nodeStates.put(index, ns);
                }
            } else {
                upNodes.set(index);
                if ( ! equalsWithDescription(defaultUp(), ns)) {
                    nodeStates.put(index, ns);
                }
            }
        }
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