This change removes all references to self.__class__. In object
hierarchies with multiple levels of inheritance, using self.__class__
can cause an infinite loop when resolving references to parent classes.
This change is primarily a reorganization of the message payload code
originally found in kmip/core/messages/operations.py. The code from that
module has been moved to a new package, kmip.core.messages.payloads, and
split into separate modules according to each supported KMIP operation.
The surrounding KMIP codebase has been updated to use the new package
structure and a placeholder test suite has been put in place for future
tests. This change does NOT include test cases as these would primarily
be a refactoring from the current test suite found in
kmip/tests/core/messages/test_messages.py and that work has not been
done yet.