instancestorage: ensure that the cache goroutine shuts down gracefully

`(*stop.Stopper).AddCloser` registers a closer to be called after all
async tasks have exited. The cache was running, waiting to be closed.
We instead need to hook up its context to exit when the stopper is
quiescing.

Epic: none

Release note: None

pkg/sql/sqlinstance/instancestorage/instancereader.go

@@ -78,17 +78,19 @@ func (r *Reader) Start(ctx context.Context, self sqlinstance.InstanceInfo) {
 			timestamp:  hlc.Timestamp{}, // intentionally zero
 		},
 	})
+	// Make sure that the reader shuts down gracefully.
+	ctx, cancel := r.stopper.WithCancelOnQuiesce(ctx)
 	err := r.stopper.RunAsyncTask(ctx, "start-instance-reader", func(ctx context.Context) {
 		cache, err := r.storage.newInstanceCache(ctx)
 		if err != nil {
 			r.setInitialScanDone(err)
 			return
 		}
-		r.stopper.AddCloser(cache)
 		r.setCache(cache)
 		r.setInitialScanDone(nil)
 	})
 	if err != nil {
+		cancel()
 		r.setInitialScanDone(err)
 	}
 }