@Override
        public UsableBroker next() {
            if (!hasNext()) {
                throw new NoSuchElementException();
            }
            UsableBroker result = next;
            next = null;
            return result;
        }
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