Enhancement: Atomic uploads for the SFTP backend

The SFTP backend did not upload files atomically. An interrupted upload could
leave an incomplete file behind which could prevent restic from accessing the
repository. This has now been fixed and uploads in the SFTP backend are done
atomically.

https://github.com/restic/restic/issues/3003
https://github.com/restic/restic/pull/3524
