Remove necessity to call create_callback_map (as well as the function
itself) from the Python P2P test framework. Invoke the appropriate
methods directly.
- Easy to forget to call it and wonder why it doesn't work
- Simplifies the code
- This makes it easier to handle new messages in subclasses
Github-Pull: #7171
Rebased-From: 2f601d215da1683ae99ab9973219044c32fa2093
