Use new event logs for site metrics

Created by: nicksnyder

#5486 (closed) introduces a new codepath to store events in the database. The end result is that metrics data is being dual written to Redis (old codepath) and Postgres (new codepath), but reads are still being served by Redis.

This issue tracks the work to add support for computing metrics based on the event log in Postgres.

• Introduce new experimental feature flag that controls whether metrics are read from Redis or computed from the event log in Postgres.
• Update existing usagestats package (or create a new package) to read from Postgres when the above feature flag is enabled.

cc @dadlerj as PM and @tsenart as code reviewer