Send updated at to rummager

I'm not familiar enough with this code to know - where does the value of updated_at come from, here?  Is this from content-store?

If not, I worry we'll get confusing situations where updated_at for a document has two different values; the idea ultimately is to use the updated_at field from content store for versioning updates in rummager (so an update with a lower updated_at field than existing content would be ignored, assuming that messages had arrived at rummager out-of-order).

This comment may make no sense out of content - please talk to me if so.