Draft: fix CheckRepoStats and reuse it during migration
The CheckRepoStats function missed the following counters:
- label num_closed_issues & num_closed_pulls
- milestone num_closed_issues & num_closed_pulls
The update SQL statements for updating the repository num_closed_issues & num_closed_pulls fields were repeated in three functions (repo.CheckRepoStats, migrate.insertIssues and models.Issue.updateClosedNum) and were moved to a single helper.
The UpdateRepoStats is implemented and called in the Finish migration method so that it happens immediately instead of wating for the CheckRepoStats to run.