model: Add new columns to Lister model related to priority scheduling

In order to implement a new scheduler runner that will schedule first
visits of listed origins with high priority, add the following new
columns to the Lister model:

- last_listing_finished_at: Timestamp at which the last execution of
  the lister finished

- first_visits_queue_prefix: Optional prefix of message queue names
  to schedule first visits with high priority

- first_visits_scheduled_at: Timestamp at which all the first visits
  of listed origins with high priority were scheduled

Related to #4687.