Draft: Add feature flag to await subprocesses in finalsink

This adds a flag that will force the pipeline to wait until the marginalized stats calculating process finishes running before proceeding. It should cause longer runtimes in exchange for more reproducible results.

We can use this for doing background runs with full determinism.

Draft

It's a draft because I'm still testing, and it may not be necessary for the followup: dumping all background stats to a single folder.