fix: unexpected errors during upload races

[benmcclelland]

This fixes the cases for racing uploads with the same object names. Before we were making some bad assumptions about what would cause an error when trying to link/rename the final object name into the namespace, but missed the case that another upload for the same name could be racing with this upload and causing an incorrect error.

This also changes the order of setting metadata to prevent accidental setting of metadata for the current upload to another racing upload.

Fixes #854