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.

  Now that cleanup actions don't occupy space in the main worker queue,
  all that needs to be done is make checksum verification be done as the
  cleanup action. Currently, it's bundled into the same action that
  transfers content.

* Using -J can sometimes lead to a slowdown while a rsync special remote
  runs Remote.Rsync.rsyncTransport, which sets up a ssh connection to the
  remote. This is done even when the remote is not used.