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.
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