feat: new DLB health check endpoint

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What does this MR do?

This PR adds a new health check endpoint to expose the health of the DB load balancer mechanism and its replicas. The new API endpoint will be located at /debug/health/db and it exposes information in the following JSON format:

{
  "overall_status": "healthy" | "unhealthy",
  "primary": {
    "address": "string",
    "status": "online" | "unknown" | "unreachable",
    "last_pinged_at": [timestamp]
  }
  "replicas": [
    {
      "address": "string",
      "status": "online" | "quarantined" | "unknown" | "unreachable",
      "quarantined_at": [timestamp]
      "last_pinged_at": [timestamp]
    },
    ...
  ]
}

In the background, I have created a new DBStatusChecker which pings the replicas to update the status for the API endpoint. To reduce the load on the database and replicas, DBStatusChecker is also responsible for the regular DB health check (previously checks.DBChecker). All of this is done asynchronously to avoid blocking API calls waiting for the ping to return.

I've added some unit tests:

• to check the DB health check works as expected
• to test the HTTP handler (status code and headers)
• to test the format of the response payload
• to check the status is returned as expected by the (synchronous) getStatus method
• to test for race conditions in the concurrency logic

Related to #1602 (closed)

QA steps

Follow the guide here to set up load balancing with GDK.

I used the following registry config:

version: 0.1
storage:
  filesystem:
    rootdirectory: bin/storage
database:
  enabled: true
  host: /home/gdk/gitlab-development-kit/postgresql/
  port: 5432
  user: gdk
  password: gitlab
  dbname: registry
  sslmode: disable
  loadbalancing:
    enabled: true
    hosts:
      - /home/gdk/gitlab-development-kit/postgresql-replica/
redis:
  cache:
    enabled: true
    addr: /home/gdk/gitlab-development-kit/redis/redis.socket
http:
  addr: :5000
  debug:
    addr: :5001
health:
  database:
    enabled: true

Once the registry is running, visit localhost:5001/debug/health/db. It might take a minute for the status of the replicas to initially update.

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    • If adding new queries, extract a query plan from postgres.ai and post the link here. If changing existing queries, also extract a query plan for the current version for comparison.
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