Fixes Timestamp Nodes When Running with --debug

[mdemoret-nv]

If the user runs with --debug, then any stage that derives from MultiMessageStage, will insert columns with the message timestamp for when it was finished processing. With some of the recent changes to Neo, we disabled broadcast nodes. However, the timestamp code didnt get updated.

This PR fixes a few things:

1. It fixes the timestamp node to return the input object instead of None
2. It updates all nodes to require overriding supports_cpp_node
   1. This is necessary to ensure _build_cpp_node() returns the correct thing
3. It only inserts timestamp nodes when C++ is disabled (we should have a C++ version instead).
   1. This uses _build_cpp_node() to determine whether C++ is being used for this particular stage
4. It updates all nodes to use _build_cpp_node() instead of CppConfig.get_should_use_cpp()

Fixes #143

[dagardner-nv]

LGTM

I suspect that the switch away from using CppConfig directly left some unused imports that the check stage is going to fail on.

[mdemoret-nv]

@gpucibot merge