= Problem

The Github API does not provide a way to fetch all the pull requests
reviews of a project (repo), like it provides for comments, instead we
have to fetch the reviews by Pull Request.

For this reason, the
Gitlab::GithubImport::Importer::PullRequestsReviewsImporter¹ have to
iterate over the imported pull requests and for each one do request the
reviews, which might be more than one page.

If the importer hits a rate limit, the process restarts, and the
imported pull requests are skipped², but the importer goes over all the
review pages again.

In other words, for some projects with large number of pull requests and
large number of reviews per pull request, we might end up with
duplicated reviews and unnecessary API requests, which would lead to
longer importing times.

= Proposed solution

- To avoid duplicated comments, besides caching the Pull Requests ids,
  also cache the review ids and skip the already processed ones.

- To avoid unnecessary API requests, use the PageCounter to only request
  pages that weren't yet imported.

Related to: https://gitlab.com/gitlab-org/gitlab/-/issues/331315
Changelog: changed
MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/62036