Filter out struct/list features for pivots

I know the code was already doing it, but this copying and setting of the tombstone value isn't really right. The output format should have a single tombstone value (not one per feature) and it means a different thing than the ingestion tombstone that is stored in the dictionary. 

I would go so far as to say we should make rename all the code to do with the output format to use "missing" or "not-set" value rather than tombstone which is related to deletion on ingest. And take aways this copy call.