server: Don't lock all awaited database Payments at once in update task

[ezdac]

with_for_update will (probably?) lock all the corresponding entries (which are all that are relevant for the WSGI backend).
Check wether this is a viable solution to lock all entries at once on a task that is recurring relatively often (on every poll of the Raiden API).

https://github.com/raiden-network/paywall/blob/c84e5407f60e5316dd477ca4a9bbf89ad2749c00/server/src/raiden_paywall/tasks.py#L17-L24