Calculate fee amount after output addition

Calculate fee amount after output addition

We would previously calculate the fee amount in two steps:
1. Add the weight of the empty transaction
2. Add the weight of each output

That's unnecessary: you can just use the weight of the transaction
*after* the output addition. This is clearer, but also avoids a
rare bug: if there are many outputs, adding them would cause the
"number of outputs" transaction parameter lenght to increase, and we
wouldn't notice it.
This might still happen when adding the drain output - this
commit also adds a comment as a reminder.