[CT-1644] Cleanup node classes as part of protobuf node generation

[gshank]

As part of #6391, we are cleaning up the node classes in core/dbt/contracts/graph/nodes.py. A previous effort combined "Parsed" and "Compiled" classes. This effort will cleanup the relationship of the various node classes to make them more consistent and the code easier to read and understand.

This will include attempting to remove attributes that should belong to particular nodes, and moving some attributes around.