Iterator<UsableBroker> usableBrokers(
        Function<Integer, Optional<String>> idToRack
    ) {
        return new UsableBrokerIterator(brokers.values().iterator(),
            idToRack);
    }
Check if the oldest broker to have heartbeated has already violated the
sessionTimeoutNs timeout and needs to be fenced.

@return      An Optional broker node id.
metadata/src/main/java/org/apache/kafka/controller/BrokerHeartbeatManager.java