Prevent duplicate insert race within createNewPaymentWithTransaction by enforcing uniqueness of _id comprising the combination of invoiceId and txid.
 Instead of inserting duplicate records (visible in test2), it results in a Document update conflict.
  TODO: Later hide non-fatal errors.  For now they are helpful in debugging.
 This non-random payment _id is planned for the next Payment model, but this use is compatible with the current old model.
 This also makes less noisy the other more common "Document update conflict" in updatePaymentWithTransaction.