Use Structured JSON logging for DB load balancing

Use structured JSON logging for ee/lib/gitlab/database/load_balancing.

We currently log when:

• A host is marked as offline
• A host is (still|came back) online
• Primary is used because there were no secondaries

This will allow us to have views into what hosts each application server thinks is still online. (This is achieved by having fluentd consume the logs as outlined in https://docs.gitlab.com/ee/development/logging.html#additional-steps-with-new-log-files)