void addNodeState(Node node, NodeState ns) {
            validateInput(node, ns);
            int index = node.getIndex();
            updateMaxIndex(index + 1);
            setNodeStateInternal(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