Add rollback last output functionality

Note that this does not include the transaction finalization which is
part of the rollback last output procedure in the opentransactions
wiki.
So of course the relationship is what defined by what the algorithm is expected to do. I haven't found names that reflect that well yet - the tx* reflects values of the inputs and outputs that should be left in the tx after the rollback. The withdrawal* reflects the stuff that was removed by the rollback.