Decrease the number of transaction body generations

[mmontin]

Balancing generates as many tx bodies as the number of iteration is the dychotomic search process. Maybe this can be reduced by generating a tx body at first and then only tampering with the elements that do change in the balancing process (inputs, outputs, fees and collaterals). We need to see if this can be done in practice.Also this could go a long way into have the balancing process return the transaction body it created, if any.