Before this change, server-side copying a src file over a dst that already exists
gave `Error "item_name_in_use" (409): Item with the same name already exists`.
This change fixes the error by copying to a temporary name first, then moving it
to the real name.
There might be a more graceful way to overwrite a file during a copy, but I
didn't see one in the API docs.
https://developer.box.com/reference/post-files-id-copy/
In the meantime, this workaround is better than a critical error.
This should (hopefully) fix 8 bisync integration tests.
3c7ad8d961