Start work on download_photo_urls.py
More work on download_photo_urls.py
Get photo URL downloads working
Download property photo URLs and update them in the database

Includes:

 - Work on database updating framework
Rename download_photo_urls.py to update_photo_urls.py

Some mild refactoring
Populate properties.last_image_transaction_when

Includes:

 - Support update_photo_urls.py without --limit option
Work on update_db_properties.py

Includes:

 - update_photo_urls.py is obsolete for the time being

 - Remove common.with_cursor to discourage cursor reuse
Download the primary photo URL as part of update_db_properties.py