Ensure all pieces come from the same object while do multi-part downloading.

[shileiyu]

I have done a quick search against the latest code on develop branch, the IFMatch prediction wasn't used in multi-part downloading task.
if someone patches or replaces the file during downloading(large file or slow network), the result may be end up as a mix-in of multiple revisions, especially when chunk size is small.

[jamesls]

Just to clarify, the idea is to take the ETag value we get during the head_object call and pass it through all the get_object() calls. If we get a 412 error then we mark the object as a failed (or retry again from the beginning). Is that right?