server: Fix possible UNIQUE constraint integrity error on get_or_create

[ezdac]

There seemed to be an edge-case where the get_or_create query caused a "create" action
although the entry was already present. This was during testing with the production setup, using multiple threads.
See if the error can be reproduced.

https://github.com/raiden-network/paywall/blob/c84e5407f60e5316dd477ca4a9bbf89ad2749c00/server/src/raiden_paywall/flask_raiden.py#L58-L70