`fraudserve.Service` should stop joined PubSub topics on `Stop`

[Wondertan]

This would be a simple 3 loc change, but it breaks our tests because PubSub does not allow closing topics if there are open subscriptions in them. They are open because they are closed asynchronously by context and there is no way to await when it's done or not. The solution is refactoring of circuit breaking described in #2041