public void handleFleetData(Map<Integer, Integer> data) {
        verifyInControllerThread();
        context.log(logger, Level.FINEST, "Sending fleet data event on to master election handler");
        metricUpdater.updateMasterElectionMetrics(data);
        masterElectionHandler.handleFleetData(data);
    }
This function gives data of the current state in master election.
The keys in the given map are indices of fleet controllers.
The values are what fleetcontroller that fleetcontroller wants to
become master.
<p>
If more than half the fleetcontrollers want a node to be master and
that node also wants itself as master, that node is the single master.
If this condition is not met, there is currently no master.
clustercontroller-core/src/main/java/com/yahoo/vespa/clustercontroller/core/FleetController.java