public String toString() {
        return String.format("state '%s' at time %d", getStateString(BASELINE), time);
    }
Represents the absolute cluster states (baseline + derived) for a given version
as well as the relative diffs (baseline + derived) since the previous version.
clustercontroller-core/src/main/java/com/yahoo/vespa/clustercontroller/core/ClusterStateHistoryEntry.java