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:

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

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

Frontend

• Shopping cart
• 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
• 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 #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...