Accept multipart uploads where the combined checksum provides the expected part count.
It seems this was added by AWS to make the API more consistent, even if the
data is entirely superfluous on multiple levels.
Improves AWS S3 compatibility.
Create new code paths for multiple subsystems in the code. This will
make maintaing this easier later.
Also introduce bugLogIf() for errors that should not happen in the first
place.
