Deduplicating transaction generation

[mmontin]

At the moment we generate the transaction twice:

• during balancing
• afterwards, in validateTxSkel

We could imagine that that the balancing could return the generated transaction so that the second generation is no longer needed. However, due to balancing option there are cases when this generation does not happen during balancing. In those cases, we could imagine two possibilities:

• generating a transaction during balancing regardless of the given options
• having balancing return a Maybe Tx and possibly generate the transaction in the validation process when equal to Nothing.