Drop of the downstream messages

[lexfrei88]

• The issue is present in the latest release.
• I have searched the issues of this repository and believe that this is not a duplicate.

What happened?

Only one of multiple downstream messages is handled if they will appear in MQTT with a delay of less than 100ms (all messages to the same device)

What did you expect?

All downstream message handled

The issue I believe in here mqtt.go#txPayloadHandler:

	// Since with MQTT all subscribers will receive the downlink messages sent
	// by the application, the first instance receiving the message must lock it,
	// so that other instances can ignore the message.
	key := fmt.Sprintf("lora:as:downlink:lock:%d:%s", pl.ApplicationID, pl.DevEUI)
	set, err := storage.RedisClient().SetNX(key, "lock", downlinkLockTTL).Result()
	if err != nil {
		log.WithError(err).Error("integration/mqtt: acquire lock error")
		return
	}

Two messages will be considered different only if they send to different devices. Why this code can't take into account the content of these messages? That would help to avoid duplications but make it possible to process different messages that came too often

Component	Version
Application Server	v3.10.0

[brocaar]

Yes, I agree. Would you like to create a PR for this?