wallet: refactor txToOutputs to use walletdb.Update
wallet: refactor txToOutputs to use walletdb.Update

To make sure we don't create any manual DB transactions, we refactor the
txToOutputs method to use walletdb.Update and the new
walletdb.ErrDryRunRollBack error for making sure a rollback is issued.

btcsuite/btcwallet,btcsuite/btcwallet