fix bug: Under high concurrency, Mqtt client nextMessageId exceeds the 0xffff limit (#2564)

The compareAndSet method and getAndIncrement method of the AtomicInteger class are atomic, but when these two methods are used at the same time, they are no longer atomic.

@@ -407,8 +407,12 @@ final class MqttClientImpl implements MqttClient {
     }
 
     private MqttMessageIdVariableHeader getNewMessageId() {
-        this.nextMessageId.compareAndSet(0xffff, 1);
-        return MqttMessageIdVariableHeader.from(this.nextMessageId.getAndIncrement());
+        int messageId;
+        synchronized (this.nextMessageId) {
+            this.nextMessageId.compareAndSet(0xffff, 1);
+            messageId = this.nextMessageId.getAndIncrement();
+        }
+        return MqttMessageIdVariableHeader.from(messageId);
     }
 
     private Future<Void> createSubscription(String topic, MqttHandler handler, boolean once, MqttQoS qos) {