code: JournalSegment<E> getNextSegment(long index) {
      Map.Entry<Long, JournalSegment<E>> nextSegment = segments.higherEntry(index);
      return nextSegment != null ? nextSegment.getValue() : null;
    }
docs: Returns the segment following the segment with the given ID.
  
  @param index The segment index with which to look up the next segment.
  @return The next segment for the given index.