Make streaming map-reduce use atomic move_dir

This is essentially using the code introduced in spotify/luigi#393,
solving the exact same issue described there.

Without this patch, the renaming is of LBYL nature, and you could end up
with moving a directory inside another instead of actually renaming a
directory.

So others don't fall into the same pitfall, I added a comment to the
rename and move function discouraging it to be used for directories.