Replace redis-based usage stats with postgres-based stats

Created by: dadlerj

fixes https://github.com/sourcegraph/sourcegraph/issues/5542

This update removes the existing usagestats package that uses redis for logging site usage statistics and replaces it with one that uses the new event_logs postgres table.

Using this new data store will make our metrics less brittle, and will allow us to show more useful information to site administrators.

It's not yet 100% clear to me if we'll ultimately need to retain (or bring back) at least some subset of the redis-based metrics as well... It depends on how well the new postgres-based metrics backend scales to our largest customers (since we need to provide basic metrics even if that's the case).