Websocket closing and handler race condition

[avanaur]

When a handler is set to return true, it seems it will remove the function and the websocket will unsubscribe to channel if no more handler for same channel.

But if the websocket is closed right away after the handler execution, the unsubscription to channel will fail and results to panic.

	conf, err := sdk.NewConfig([]string{"http://csddev1.xpxsirius.io:3000", "http://csddev2.xpxsirius.io:3000"}, sdk.PrivateTest, sdk.WebsocketReconnectionDefaultTimeout)
	if err != nil {
		panic(err)
	}

	ctx, cancel := context.WithTimeout(context.TODO(), time.Second*30)
	//defer cancel()
	wsClient, err := websocket.NewClient(ctx, conf)
	if err != nil {
		panic(err)
	}
	go wsClient.Listen()

	address, err := sdk.NewAddressFromRaw("WAFSWQIF2UBLBHOFOYMIQJXZKRSTNONZIM6VYAZT")
	if err != nil {
		panic(err)
	}

	txn, err := sdk.NewTransferTransaction(
		sdk.NewDeadline(time.Hour),
		address,
		[]*sdk.Mosaic{},
		sdk.NewPlainMessage(fmt.Sprintf("hi there %d", rand.Int())),
		sdk.PrivateTest,
	)
	if err != nil {
		panic(err)
	}

	acct, err := sdk.NewAccountFromPrivateKey("b007b006c2178a8819c6b38dd3b49db1c2170483fdfcff21656f071c7541b5a9", sdk.PrivateTest)
	if err != nil {
		panic(err)
	}

	signedTxn, err := acct.Sign(txn)
	if err != nil {
		panic(err)
	}

	var wg sync.WaitGroup
	wg.Add(1)
	err = wsClient.AddUnconfirmedAddedHandlers(address, func(txn sdk.Transaction) bool {
		fmt.Println("FOUND", txn.GetAbstractTransaction().Hash.String())
		if txn.GetAbstractTransaction().Hash.String() == signedTxn.Hash.String() {
			fmt.Println("FOUND HASH IT ON UNCONFIRMED", txn.GetAbstractTransaction().Hash.String())
			wg.Done()
			// ISSUE
			return true
		}
		return false
	})
	if err != nil {
		panic(err)
	}

	client := sdk.NewClient(nil, conf)
	_, err = client.Transaction.Announce(context.Background(), signedTxn)
	fmt.Println("ANNOUNCED", signedTxn.Hash)
	if err != nil {
		panic(err)
	}

	wg.Wait()

        cancel()