Prevent opening success page multiple times

[frank-bokdam]

Thank you for creating this pull request! To make the best use of your and our time we created this checklist to get the best possible pull requests:

• [x] The code is working on a plain Magento 2 installation.
• [x] The code follows the PSR-2 code style.
• [x] When an exception or error is logged the message is accompanied with some context, eg: Error when trying to get the payment status:
• [x] Contains tests for the changed/added code (great if so but not required).
• [x] I have added a scenario to test my changes.

This PR touches code in the following areas (Check what is applicable):

Frontend

• [ ] Shopping cart
• [x] Checkout
• [ ] Totals
• [ ] Payment methods

Backend

• [ ] Configuration
• [ ] Order grid
• [ ] Order view
• [ ] Invoice view
• [ ] Credit memo view
• [ ] Shipment view
• [ ] Email sending

Order Processing (Mollie communication)

• [ ] Creating the order
• [x] Invoicing the order
• [ ] Shipping the order
• [ ] Refunding (credit memo) the order

Other If you didn't check any boxes above, please describe your changes in this section.

Please describe the bug/feature/etc this PR contains:

Fix for https://github.com/mollie/magento2/issues/687

The idea is to check if the payment was already processed. If true, it makes no sense to reset checkout session data.

Scenario to test this code:

Open the environment and...