private void updateWorkingState(Node node, boolean isWorking) {
        log.fine(() -> "Updating working state of " + node + " to " + isWorking);
        node.setWorking(isWorking);
        updateVipStatusOnNodeChange(node, isWorking);
    }
Returns the single, local node we should dispatch queries directly to,
or empty if we should not dispatch directly.
container-search/src/main/java/com/yahoo/search/dispatch/searchcluster/SearchCluster.java