code: public Iterable<Location> redo() throws ClosedJournalException, CompactedDataFileException, IOException {
          Entry<Integer, DataFile> firstEntry = dataFiles.firstEntry();
          if (firstEntry == null) {
              return new Redo(null);
          }
          return new Redo(goToFirstLocation(firstEntry.getValue(), Location.USER_RECORD_TYPE, true));
      }
docs: Return an iterable to replay the journal by going through all records
  locations.
  
  @return
  @throws IOException
  @throws ClosedJournalException
  @throws CompactedDataFileException