add custom fee, fix position of incoming_tx #28

This PR allows the setting of custom fees for transactions in sdl. It also fixes a problem with the position of the incoming_metadata, which is why some users could not see some messages from the mempool.

@lucretius I have a question about this code:

        for (_, _, _, fee) in send_args.iter() {
            if *fee != default_fee {
                total_fee = *fee;
                break;  
            }
        }

If it's calculating a "total fee" should that be += instead of = ?

@lucretius I have a question about this code:
        for (_, _, _, fee) in send_args.iter() {
            if *fee != default_fee {
                total_fee = *fee;
                break;  
            }
        }
If it's calculating a "total fee" should that be += instead of = ?

@duke
The name total_fee might be misleading since the logic of the code is not actually calculating a "total" of all fees. Instead, it is finding the first fee value that differs from the default and using that as the fee for the operation. A more appropriate name for the variable might be applicable_fee or selected_fee, as it better reflects its purpose in the code. You are right

@duke i changed the name to selected_fee to better reflect what it is for, and i fixed a copy pasta error in the name (superfluous ,)