Replace redis-based usage stats with postgres-based stats
Created by: dadlerj
fixes #5542
(This PR is the second piece of https://github.com/sourcegraph/sourcegraph/pull/6473, after it was broken up)
This update adds a new usagestats package that uses the new event_logs
Postgres table. It deprecates the old usagstats package that uses Redis.
Using this new data store will make our metrics less brittle, and will allow us to show more useful information to site administrators.
I had originally structured this to REMOVE the old package entirely, but I decided to keep it for now. The compute cost is low, the old package can be removed next month, and having overlapping metrics from each will be helpful for ensuring validity for site admins.