Major Description
When OfflineSorter needs to write a new file, either a temp file or the final sorted output file, it calls getWriter. There is a default impl, but experts can override it to create their own writers.
It would be helpful to pass the total item count that will be written to this file when calling getWriter because then the (expert) user has a little more information which can be useful when creating the temp or final file.
It's low cost for OfflineSorter to track this.