add txn purging to the renter

If negotiation failed at some point, the transaction was left in limbo - outputs had been consumed according to the wallet, but actually had not been consumed since the transaction had failed. The transaction builder now includes a purge function that allows for outputs to be 'dropped' from a transaction builder in the event of a failed transaction. And the renter will now purge any in-progress transaction if negotiation fails, so that any outputs can be restored.