When a multipart S3 upload is being made, and gets interrupted,
the parts remain in the bucket, and S3 may charge for them.

I am not sure what happens if the same object gets uploaded again. Is S3
nice enough to remove the old parts? I need to find out..

If not, this needs to be dealt with somehow. One way would be to configure an
expiry of the uploaded parts, but this is tricky as a huge upload could
take arbitrarily long. Another way would be to record the uploadid and the
etags of the parts, and then resume where it left off the next time the
object is sent to S3. (Or at least cancel the old upload; resume isn't
practical when uploading an encrypted object.) 

It could store that info in either the local FS or the git-annex branch.
