In the C++ API there are four overloads of the addDagCallback()
method. Two of them register a callback that just takes the child node, the
parent node and the client data as input whereas the other two register
a callback that receives the DAG message as well.
In this Python binding these two groups are split into two methods.
The addDagCallback() method registers the callback that receives
the message whereas addDagCallback2() registers
the callback without the message argument.
