Prior to Swift commit 7f636a557296ecc6ae4727700cfcf9f82573bd16, SLO
would allow large objects to include zero-byte segments if they were the
last segment in the manifest. This was consistent with AWS's behavior
during multipart uploads.
Now, however, SLO requires that every segment be at least one byte. This
commit once more allows zero-length final parts by removing such parts
from the manifest and cleaning up the already-uploaded segment.
Additionally, if the multipart upload consisted of a single zero-length
part (which is apparently allowed via S3), an ordinary object will be
created instead of a SLO.
Change-Id: I06750b2ffcb2a90bee44fa366afda4520705f09c
eb97e176f1