Function transition_unless_moved_on 

pub async fn transition_unless_moved_on(
    conn: &mut PgConnection,
    id: Uuid,
    transition: &Transition,
) -> ModelResult<Transitioned>

transition for a caller deciding from a snapshot another writer may have overtaken: a row no longer in expected_from_state comes back as Transitioned::MovedOn rather than as an error, since the row is now that writer’s and the caller carries on with the rest of its work.