public Set<CoordinatorStreamMessage> getUnreadMessages(SystemStreamPartitionIterator iterator) {
    return getUnreadMessages(iterator, null);
  }
returns all unread messages after an iterator on the stream

@param iterator the iterator pointing to an offset in the coordinator stream. All unread messages after this iterator are returned
@return a set of unread messages after a given iterator
samza-core/src/main/java/org/apache/samza/coordinator/stream/CoordinatorStreamSystemConsumer.java