spotted while reviewing #15303

In the Pubsub, we can't hold the qMu lock while calling Listen or Unlisten on the pgListener, because this lock is also held while delivering notifications. This was the source of a Pubsub deadlock (#11950) that we fixed in #12518. However, that fix leaves us with a new race condition that goes like this:

We have a single subscriber to some event, and they cancel their subscription. Concurrently, we get a new Subscribe call for the same event. Since we don't hold a lock while calling Listen and Unlisten, these could happen in any order. If Unlisten is called last, the new subscriber won't get any events.