Finished this feature, and I'm liking it quite a lot!
Though I had to put off [[todo/support_public_versioned_S3_access]] until
some other time as I'm all out of time and energy.

The storage of S3 version IDs got rethought -- I was not comfortable with
using per-remote state in the git-annex branch which would have caused
problems if dropping from these remotes later gets supported.

So, I added per-remote metadata in about 1 hour! It's like git-annex's regular
metadata, but scoped so only the remote that owns it can see it. This is
perfect for storing things like S3 version IDs. It probably ought to be added
to the external special remote interface since it could be used for lots of
stuff.

Here's how that looks when S3 version IDs are stored in it:

	1535737778.867692782s 31ea6c94-fba3-4952-99b5-285ae192d92a:V +woYHK59DD2VUkJfg527mEBBqtCaPlSXn#myfile

This work was supported by the NSF-funded DataLad project.
