Add a judgment on QoS in the publish method

[xyangwang]

I use this component as a broker, and use the eclipse paho client receive message from broker.
I found that using the MqttEndpoint publish method to send messages to clients, regardless of whether the QoS is 0 or not, each time it get the next message ID. But if paho client receive a message with QoS 0, its message ID is 0.

MqttPublishVariableHeader variableHeader =
      new MqttPublishVariableHeader(topic, this.nextMessageId());

I write an example, one client send a message with QoS 0 and next time send a message with QoS 1 to the other, loop this function 10 times.

for (int i = 0; i < 10; ++i) {
      mqttClient.publish(testTopic, "hello".getBytes(), 0, false);
      mqttClient.publish(testTopic2, "world".getBytes(), 1, false);
}

the receive client console log:

mqtt client is connected.
----connect complete-----
false : tcp://127.0.0.1:1886
message id = 0, qos = 0, payload = hello
1
message id = 2, qos = 1, payload = world
2
message id = 0, qos = 0, payload = hello
3
message id = 4, qos = 1, payload = world
4
message id = 0, qos = 0, payload = hello
5
message id = 6, qos = 1, payload = world
6
message id = 0, qos = 0, payload = hello
7
message id = 8, qos = 1, payload = world
8
message id = 0, qos = 0, payload = hello
9
message id = 10, qos = 1, payload = world
10
message id = 0, qos = 0, payload = hello
11
message id = 12, qos = 1, payload = world
12
message id = 0, qos = 0, payload = hello
13
message id = 14, qos = 1, payload = world
14
message id = 0, qos = 0, payload = hello
15
message id = 16, qos = 1, payload = world
16
message id = 0, qos = 0, payload = hello
17
message id = 18, qos = 1, payload = world
18
message id = 0, qos = 0, payload = hello
19
message id = 20, qos = 1, payload = world
20

I think it's necessary to add a judgment on QoS in the publish method to prevent the client from receiving discontinuous message IDs.