Improve Import/Export memory use and performance

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/35389

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/26556

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together