fix Deprecated Warning

[dushibaiyu]

• messageId
• password
• willMessage

[Sammers21]

@dushibaiyu , sorry, I did not get it. Which warning are you trying to fix? Can you explain?

[Sammers21]

@dushibaiyu , this PR also contains some parts from the #101. Can you remove it, so the PR will contain only original changes?

[dushibaiyu]

@Sammers21 Ok. i have remove the #101 commit.

• In class MqttPublishVariableHeader the messageId() is deprecated it return the packetId.
• In class MqttConnectPayload the willMessage() and password() is deprecated .

[ppatierno]

With this PR I would just fix the deprecated warnings for messageId. I think that will message and password should have a different PR because ...
Since the first MQTT implementation in Netty, it was a big mistake having will message and password as String because they are just bytes in the MQTT specification. Now that it was addressed in Netty, I think that we should have a breaking change here having will message and password changing from String to bytes in the MQTT library as well. I know that Netty has is maintaining its back compatibility with fields like willMessage and willMessageInBytes with the first one having the bytes encoded in UTF-8 but the first one doesn't make sense if the will message is a really raw binary message with no meaning as String. Wdyt @vietj , is that bad having a breaking change here? I mean exposing changing from String to bytes without having both exposed?