Unregister ObserveMultiplexer before stop

It's maybe possible that stop() yields, and in general is more complex.
Since the ObserveMultiplexer is already at the point where it will not
be able to start running again, we should remove it from the registry
first.

packages/mongo/mongo_driver.js

@@ -1094,8 +1094,8 @@ MongoConnection.prototype._observeChanges = function (
       multiplexer = new ObserveMultiplexer({
         ordered: ordered,
         onStop: function () {
-          observeDriver.stop();
           delete self._observeMultiplexers[observeKey];
+          observeDriver.stop();
         }
       });
       self._observeMultiplexers[observeKey] = multiplexer;