git-annex has good support for running commands in parallel, but there
are still some things that could be improved, tracked here:

* Maybe support -Jn in more commands. Just needs changing a few lines of code
  and testing each.

* Maybe extend --jobs/annex.jobs for more control. `--jobs=cpus` is already
  supported; it might be good to have `--jobs=cpus-1` to leave a spare
  cpu to avoid contention, or `--jobs=remotes*2` to run 2 jobs per remote.

* Parallelism is often used when the user wants to full saturate the pipe
  to a remote, since having some extra transfers running avoid being
  delayed while git-annex runs cleanup actions, checksum verification,
  and other non-transfer stuff.

  But, the user will sometimes be disappointed, because every job
  can still end up stuck doing checksum verification at the same time, 
  so the pipe to the remote is not saturated.

  Running cleanup actions in a separate queue from the main job queue
  wouldn't be sufficient for this, because verification is done as part
  of the same action that transfers content. That needs to somehow be
  refactored to a cleanup action that ingests the file, and then
  the cleanup action can be run in a separate queue.