Function flush_to_db 

fn flush_to_db(
    db: &Db,
    library_id: i64,
    books_to_insert: Vec<(Fp, Info)>,
    path_updates: Vec<(Fp, PathBuf, PathBuf)>,
    books_to_delete: Vec<Fp>,
    thumbnails_to_delete: Vec<Fp>,
)