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.
This also fix auth.CheckObjectAccess() when objects are removed
while this runs.
Fixes#854
