Github importer have a mechanism to re-schedule an Importer job if it
gets throttled. But, repositories with large number of pull requests,
were in a _throttling loop_ for the Pull Request Reviews Importer.

In other words, when re-scheduled, the job was re-starting importing
reviews to all imported Pull Requests, regardless if already imported
or not, causing another throttling.

To fix this, already imported Pull Requests ids are being cached and
skipped, this way, when the job re-start, we don't do requests related
to already imported Pull Requests.

The same is applied to the Pull Request MergedBy importer.

Related to: https://gitlab.com/gitlab-org/gitlab/-/issues/329552

Changelog: changed
MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/60668