make move quicker

we saw an airbrake that showed a missing file was read and shortly before that a new download ran,
I'm suspecting the old move logic is too slow since it moves from tmp to the data (different device), so I'm adding this improvement.

https://zendesk.airbrake.io/projects/88941/groups/71699940/notices/1110146940693517314

(still not atomic, but should be close enough)

@pdeuter @bhouse @anamartinez