revisiting client lib

[tz70s]

Currently, the client library is also horrible and not usable.

To give a brief implementation tips and plan,

• Ensure the correct semantics of packages and related classes.
• Prettify
• Subscribe callback
• Remain logics modification.

In the experience of eclipse paho, we can add a subscribe callback when the message arrived.
i.e. the callback handler

public interface MessageCallBackHandler {
    void onSuccess(Message message);
    void onFailure(ErrorMessage errorMessage);
}

or we can integrate these into one function for lambda expression

@FunctionalInterface
public interface MessageCallBackHandler {
   // The message ErrorMessage will be the subtype of Message.
    void onComplete(Message message);
}

Further, the client will set up the callback handler and pass it into channel and related processor.

BlockClient blockClient = new BlockClient();
blockClient.setCallBack( new MessageCallBackHandler { /* implementation */ } );

This issue is not in a top priority, but may needed for the experiment in the future.