task/ta22-12 -- update job_item.extract_ts upon processing success

nparcel/dbsession.py
* factored out the date_now() method from nparcel/loader.py

nparcel/exporter.py
* new method _update_status() to set the job_item.extract_ts column of
  record *id* to the current date/time

nparcel/table/jobitem.py
* upd_collected_sql(self, business_unit, time) SQL wrapper to update
  the collected items from the "jobitems" table