Revisit ToC backfill mechanism

[ilijapuaca]

I ran into an issue where ToC backfill mechanism did not work properly in #342, but I do see it being brought up in a few other places. Simplifying the implementation around ToC generation would allow for quite a big chunk of complex code that handles race condition in node processing to go away, and would likely allow for easier maintenance.

One potential way to do this would be moving node processing from onCreateNode to sourceNodes, not sure if there are any practically issues with doing so, but it could be an easy transition

[ilijapuaca]

cc @ibgreen