Clean up stale journal entries and temporaries. #2057

* Downloadinfo entries for files that no longer need to be downloaded
  are useless and can be removed. In particular, the temporary files
  holding partially retrieved files are now deleted when no longer
  necessary.
* The same is true for blacklist entries for paths that are no longer
  being discovered.
* Same for uploadinfos for files that no longer need to be uploaded.
Merge pull request #2161 from ckamm/stale-downloadinfo

Clean up stale journal entries and temporaries. #2057