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.
I find it a bit confusing that the data passed to createFakeCredits() is defined in testData.want but for createWithdrawalOutputs() it is defined directlyin testData. That kind of suggests to me the split between what goes in testData.want and what goes in testData is not very clear. Maybe we could have everything in testData?