RUST: Separate connexion and queue + add todo functions + update visibilities

src/rust/liblektor-rs/lektor_db/src/connexion.rs

src/rust/liblektor-rs/lektor_db/src/lib.rs

src/rust/liblektor-rs/lektor_db/src/models.rs

src/rust/liblektor-rs/lektor_db/src/queue.rs