Before this change, Hasher would sometimes try to stop a bolt db that was already stopped, resulting in an error. This change fixes the issue by checking first whether the db is already stopped. https://forum.rclone.org/t/hasher-with-gdrive-backend-does-not-return-sha1-sha256-for-old-files/44680/11?u=nielash
Add bolt-based key-value database support. Quick API description: https://github.com/rclone/rclone/pull/5587#issuecomment-942174768