Geo: Implement GeoLogCursor for event log

The GeoLogCursor is a daemon that reads like a cursor through existing geo_log_events and process each one of them.

In this first iteration we want to handle repository and wiki update events. Whenever the cursor process an event, it checks the metadata and mark the affected repository or wiki as requiring a resync.

Are there points in the code the reviewer needs to double check?

This MR is rebased on top of gitlab-org/gitlab-ee!1976, so most of the code in the MR can be ignored as it's duplicated from other. You can check the last commit which includes the changes that are specific to this MR.

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #2467 (closed)