_timestamp updates for retraction

[sashakames]

If you are trying to search temporally using from,to you get the original publication date. However, this is less helpful if the data was retracted in the period. Retracted data have no files so can't be downloaded. Update the _timestamp to reflect the retraction date. This change will help with Synda and notification services/clients. An alternative would be to add a different field for the retraction time but that would require an additional query.