Enhancement: Create subdirs below `data/` for local/sftp backends

The local and sftp backends now create the subdirs below `data/` on
open/init. This way, restic makes sure that they always exist. This is
connected to an issue for the sftp server.

https://github.com/restic/restic/issues/1055
https://github.com/restic/rest-server/pull/11#issuecomment-309879710
https://github.com/restic/restic/pull/1077
https://github.com/restic/restic/pull/1105
