Use update instead of CreateInBatches in CompleteMultipartUpload

[ChrisSchinnerl]

This PR uses an update instead of create now when completing a multipart upload.
That's possible now since we recently added the object_index to the slices and we no longer rely on the ID for the insertion order.

This also gets rid of the separate PartialSlabs field in the object.Object since workers won't be able to deduct the right download order. Instead partial slabs are now part of the regular slabs but without any slices.

Closes #769

[peterjan]

should we mention #769 here?

[mike76-dev]

Do you think we need to have the slab order included in the object metadata?
Currently, the partial slabs are listed after the complete slabs, with no sequential order. However, with the multipart uploads it happens very often that a partial slab stands between two complete slabs. When the metadata is imported (from a satellite, or using a curl command), there is no way to assemble the slabs correctly without knowing their order.