Major issues with /pay:

1) Why is there a 'paid' field in merchant_contract_terms DB table?

2) If we do have the row_id of merchant_contract_terms, why do we store
   the full h_contract_terms/merchant_pub again and again in the other tables?

3) What happens if we store in merchant_transactions some timestamp/exchange
   and then the deposits (partially) fail? Do we undo the deposits? Do we
   allow a 2nd round of /pay? What happens if the exchange differs the 2nd time?