Improvements to Saving

* Changed saving code to create backups by moving existing files rather
than creating copies and deleting the originals.
* Removed final call to PocketManager.save() in PocketManager.unload().
Since we no longer check if the caller is the client or server and
unload() must be called from both, this prevents clients from trying to
save pocket data locally. A final save() call wasn't needed anyway.
* Changed DDLogger.logger() to private to guarantee that the logger isn't
being used anywhere in DD, since that could impact performance on
servers.