New marketplace payment handling

[jaensen]

1. Buyer fills cart with Offers from multiple Shops/Sellers
2. Buyer confirms purchase by clicking "Buy now"
3. Marketplace creates invoices for each Seller
4. Marketplace hands the invoices over to the new PaymentService
5. The PaymentService creates an EOA for each invoice (to collect all payments as a 'trustee')
6. The payment service notifies the frontend about where to send the payments to (to the EOAs)
7. On each new incoming TX the PaymentService checks if the whole payment amount arrived
8. When the whole payment amount was reached then the service sends the contents of the EOA to the seller (in one Transaction)
9. Add "payed" event to the invoice
10. Complete the purchase when all invoices have been payed

Specialties of € payments:

• Are done via GoCardless
• For every incoming GoCardless TX we will send xDai of the same worth to the payment EOA.