BulkImports: do not export relations if no updates are made since last export

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Suggestion from !84207 (comment 901184388)

When GitLab Migration runs it exports project/group relations (e.g. issues/mrs/labels) in order to import them at the destination project/group.

We recently made a change (!84207 (merged)) to not run export if it's been recently exported (3 minutes), however, @reprazent suggested to check MAX(updated_at) < export.created_at and not perform relation export if no changes were made.