trappy/ftrace: merge primary and secondary DFs based on pivot

Created by: joelagnel

I hacked something up that's working, just posting here. Basic idea is concatenate the DFS, mark them as primary/secondary, walk through them and remember the secondary information, propagate it to primary and rebuild the primary DF. I haven't written test cases yet. (CC @derkling @bjackman @sinkap)

Here's a gist showing it working in @derkling 's notebook: https://gist.github.com/joelagnel/cc08ba964e40467e828741c691011ffc