`Initialize` command should write to `site_visits`

[vringar]

Currently the TaskManager is responsible for writing into the site_visits table: https://github.com/mozilla/OpenWPM/blob/8dad9fd2129f8375575813baaa19992efdde47de/openwpm/task_manager.py#L381-L391

Steven and I agreed that it would be better to move this writing into the Storage Controller when it receives the initialize command.