Finished the refactoring that I had started on Thursday. This was only
a partial success, because it didn't result in the speedup to -J that I had
hoped for. The slow start with -J turns out to not be caused by concurrency
overhead at all, but a bug,
[[bugs/rsync_and_gcrypt_special_remotes_make_-J_slow]].

What was successful is that I got rid of the oldest implementation wart in
git-annex, the implicitMessages state. And, I made progress toward
separately parallizing checksum verification.

This refactoring is still cooking in the `starting` branch and will be
merged after the next release.
