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

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

Start with fraudserve.Service.Stop and the PubSub topic and subscription lifecycle described in the issue. Read the circuit-breaking refactor in #2041 before changing this issue, then run the affected tests; done means joined topics can be stopped without PubSub rejecting open subscriptions.

Written by the indexing model from the issue text.