app/vmstorage: prevent from serving conns from vminsert and vmselect after the server is closed

Previously it was possible that the connection is served after the server is closed if the following
steps are performed:

1) Server accepts new connection.
2) Server.MustClose() is called and successfully finished.
3) Server starts processing the connection accepted at step 1. There could be various crashes
   like in https://github.com/VictoriaMetrics/VictoriaMetrics/issues/534 since the storage may be already closed.

Now the server closes the connection at step 3 without processing it.

app/vmstorage/transport/server.go

@@ -48,18 +48,24 @@ type Server struct {
 }
 
 type connsMap struct {
-	mu sync.Mutex
-	m  map[net.Conn]struct{}
+	mu       sync.Mutex
+	m        map[net.Conn]struct{}
+	isClosed bool
 }
 
 func (cm *connsMap) Init() {
 	cm.m = make(map[net.Conn]struct{})
+	cm.isClosed = false
 }
 
-func (cm *connsMap) Add(c net.Conn) {
+func (cm *connsMap) Add(c net.Conn) bool {
 	cm.mu.Lock()
-	cm.m[c] = struct{}{}
+	ok := !cm.isClosed
+	if ok {
+		cm.m[c] = struct{}{}
+	}
 	cm.mu.Unlock()
+	return ok
 }
 
 func (cm *connsMap) Delete(c net.Conn) {
@@ -73,6 +79,7 @@ func (cm *connsMap) CloseAll() {
 	for c := range cm.m {
 		_ = c.Close()
 	}
+	cm.isClosed = true
 	cm.mu.Unlock()
 }
 
@@ -116,8 +123,12 @@ func (s *Server) RunVMInsert() {
 		}
 		logger.Infof("accepted vminsert conn from %s", c.RemoteAddr())
 
+		if !s.vminsertConnsMap.Add(c) {
+			// The server is closed.
+			_ = c.Close()
+			return
+		}
 		vminsertConns.Inc()
-		s.vminsertConnsMap.Add(c)
 		s.vminsertWG.Add(1)
 		go func() {
 			defer func() {
@@ -179,8 +190,12 @@ func (s *Server) RunVMSelect() {
 		}
 		logger.Infof("accepted vmselect conn from %s", c.RemoteAddr())
 
+		if !s.vmselectConnsMap.Add(c) {
+			// The server is closed.
+			_ = c.Close()
+			return
+		}
 		vmselectConns.Inc()
-		s.vmselectConnsMap.Add(c)
 		s.vmselectWG.Add(1)
 		go func() {
 			defer func() {