MM-30987- Fix race in setting clearNotify in Busy (#16430)

A time.AfterFunc runs the function in its own goroutine. So we need
to guard that with a mutex too.

https://mattermost.atlassian.net/browse/MM-30987

```release-note
NONE
```

app/busy.go

@@ -65,7 +65,11 @@ func (b *Busy) setWithoutNotify(dur time.Duration) {
 	b.clearWithoutNotify()
 	atomic.StoreInt32(&b.busy, 1)
 	b.expires = time.Now().Add(dur)
-	b.timer = time.AfterFunc(dur, b.clearWithoutNotify)
+	b.timer = time.AfterFunc(dur, func() {
+		b.mux.Lock()
+		b.clearWithoutNotify()
+		b.mux.Unlock()
+	})
 }
 
 // ClearBusy marks the server as not busy and notifies cluster nodes.

app/busy_test.go

@@ -86,6 +86,16 @@ func TestBusyExpires(t *testing.T) {
 	require.Eventually(t, func() bool { return compareBusyState(t, busy, cluster.Busy) }, time.Second*15, time.Millisecond*20)
 }
 
+func TestBusyRace(t *testing.T) {
+	cluster := &ClusterMock{Busy: &Busy{}}
+	busy := NewBusy(cluster)
+
+	busy.Set(500 * time.Millisecond)
+
+	// We are sleeping in order to let the race trigger.
+	time.Sleep(time.Second)
+}
+
 func compareBusyState(t *testing.T, busy1 *Busy, busy2 *Busy) bool {
 	t.Helper()
 	if busy1.IsBusy() != busy2.IsBusy() {