Although the attachment importers skip importing objects that have
already been imported, previously when restarted these importers would
start the work from the beginning again.

This change has the attachment importers resume work from where they
left off.

In cases where there are very large amounts of issues, MRs, or notes,
this will increase the chance of these importers finishing without
being interrupted due to a deploy.

This is in preparation of changing the `max_retries_after_interruption`
for all GitHub stage importers.

#416777

Changelog: performance