Turn repocheck.log into structured log (json)

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Problem to solve

Now Gitlab::RepositoryCheckLogger is a plain Gitlab::Logger, this should become a Gitlab::JsonLogger.

This might also fix gitlab-ce#44404

Further details

This will generate structured logs, making it easier to parse with Kibana.

What does success look like, and how can we measure that?

Parseable logs with Kibana :all_the_things:.

Links / references

https://gitlab.slack.com/archives/C32LCGC1H/p1525797511000073