rust-index: honour incoming using_general_delta in `deltachain` It looks to be a leftover from some past, but the C index considers only the value passed from Python whereas up to now the Rust index was using the value of its attribute. As a middle ground, we make this argument of `deltachain` optional from the Python side, with the Rust implementation only defaulting to its attribute. This way, we reduce false leads when a difference in results is spotted.