public Status halted() {
            if (state != State.RUNNING)
                throw new IllegalStateException("Current state must be RUNNING when changing to READY");
            return new Status(startedAt, null, progress, State.READY, null);
        }
Returns a copy of this in state HALTED.
clustercontroller-reindexer/src/main/java/ai/vespa/reindexing/Reindexing.java