Add helper to identify / exclude / remove (duplicate) data from unsuccessful job retries

[motin]

openwpm/OpenWPM#468 introduced job retries so that failed crawl jobs got retried n times. While this made running crawls much smoother, it also introduced data duplication, where the data for a job / crawl record / site visit could be (possibly partially) written but not recorded as successful, hence retried and resulting in a duplicated data for a site visit.

Feel free to close if you feel like it would be better to solve the root cause of the data duplication (related: openwpm/OpenWPM#476) or flag the final crawl_history entries as final instead of adding a helper.

[vringar]

This is addressed in OpenWPM by:

• Fixing crawler openwpm/OpenWPM#599
• Mark visits during shutdown as incomplete openwpm/OpenWPM#589
• On saved callback openwpm/OpenWPM#574

As a combination of those give us the incomplete_visits table.
#19 now aims to make use of that table to filter out any interrupted visits as well as ones that failed