int nextUnfenced(int epoch) {
            return unfenced.next(epoch);
        }
Get the next unfenced broker in this rack, or -1 if there are no more brokers
to be returned.

@param epoch     The current iteration epoch.

@return          The broker ID, or -1 if there are no more brokers to be
                 returned in this epoch.
metadata/src/main/java/org/apache/kafka/metadata/placement/StripedReplicaPlacer.java