Geo: Use commit timestamp to scan for events

@Finotto mentioned we might be able to improve the Geo Log Cursor by doing something like:

1. Use a PostgreSQL trigger to writing the COMMIT timestamp in a column of geo_event_log
2. Instead of scanning IDs, scan for events based on the commit timestamp.

This has the advantage of not relying on IDs appearing order: gitlab-com/migration#295 (comment 88041617)