public void setGroup(Distribution distribution) {
        this.group = null;
        if (distribution != null) {
            this.group = distribution.getRootGroup().getGroupForNode(node.getIndex());
        }
    }
When a node goes from an up state to a down state, update this flag with the start timestamp the node had before going down.
The cluster state broadcaster will use this to identify whether distributors have restarted.
clustercontroller-core/src/main/java/com/yahoo/vespa/clustercontroller/core/NodeInfo.java