Download: Don't trigger too many concurrent hash computations

Download: Don't trigger too many concurrent hash computations

Previously the job would only become "active" when the downloads
started. That meant that arbitrarily many hash computations could be
queued at the same time.

Since the the file was opened during future creation this could lead to
a "too many open files" problem if there were lots of new-new conflicts.

To change this:
- Make PropagateDownload become active when computing a hash
  asynchronously.
- Make the computation future open the file only once it gets run. This
  will make it less likely for this problem to occur even if thousands
  of these futures are queued.

For #7372