TxOut::minimal_non_dust has 3 problems.
1. There is an invisible dependency on Bitcoin Core's default minrelaytxfee value. It has been made explicit.
2. There is an off by one error. The dust limit comparison uses < and therefore `+ 1` was not needed. It has been fixed.
3. It was not returning 0 amount for OP_RETURN outputs.
Script::dust_value has 2 problems.
1. The dust amount depends on minrelaytxfee which is configurable in Bitcoin Core. This method was not configurable.
2. The division operation was done before multiplying the byte amount, which can cause small differences when using uncommon scripts and minrelaytxfee values.
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