Previously: schema specifying the download_index, and incrementing one-by-one.  Dictionary of youtube_ids was hoped to be in consistent order, and what was triggered from download would hopefully follow that same order.

Now, we still assume things, but with complete control in the view.  We assume that index 0 is the most active download.  We compute everything else from all values: the total % downloaded so far (averaging across key-values), and the # completed (counting the # of key-values = 100%