Enhancement: Verify uploaded files

The rest-server now by default verifies that the hash of content of uploaded
files matches their filename. This ensures that transmission errors are
detected and forces restic to retry the upload. On low-power devices it can
make sense to disable this check by passing the `--no-verify-upload` flag.

https://github.com/restic/rest-server/issues/122
https://github.com/restic/rest-server/pull/130