OPENDJ-2446: Improve the scalability of the dsreplication purge-historical task

* Use paging control on the internal search to reduce memory consumption of this task.
* Only run modify operations against entries if their historical attr value is really being changed (significant performance improvement).
* Amended Javadoc to describe what purgeConflictsHistorical actually does.
* lastCSNPurgedFromHist is now much more accurate. Previously it could be set to a CSN that was checked but not actually purged. This means that repeated running of the task is more reliable.
* Reset lastCSNPurgedFromHist after a full run completes successfully.