## About the changes
Adds migration for creating table `stat_traffic_usage`.
This table primary-keys on day, traffic_group, and status_code_series.
Adds individual indexes for day, traffic_group, and status_code_series.
Traffic group is the grouping for API endpoints for which traffic is
counted.
status_code_series is 200/202 etc = 200, 304 etc = 300
