private Nodes getNodes(NodeType type) {
        return (type == NodeType.STORAGE)
                ? storageNodes
                : (type == NodeType.DISTRIBUTOR) ? distributorNodes : null;
    }
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