Bugfix: Fix possible data loss due to interrupted network connections

When rest-server was run without `--append-only` it was possible to lose uploaded
files in a specific scenario in which a network connection was interrupted.

For the data loss to occur a file upload by restic would have to be interrupted
such that restic notices the interrupted network connection before the
rest-server. Then restic would have to retry the file upload and finish it
before the rest-server notices that the initial upload has failed. Then the
uploaded file would be accidentally removed by rest-server when trying to
cleanup the failed upload.

This has been fixed by always uploading to a temporary file first which is moved
in position only once it was uploaded completely.

https://github.com/restic/rest-server/pull/142