Blueprint+Design: int4->int8 conversion for primary keys

This contains the analysis of the "int4 PK" problem and design of implementation of the solution solving the problem.

Blueprint (please ensure that CI/CD delivered the most up-to-date version; check if there ara pipelines in progress): https://nik-blueprint-int4-to-int8.about-src.gitlab.com/handbook/engineering/infrastructure/blueprint/201903-postgres-int4-pk/index.html

Related issue: https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/5828

Implementation MR: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24512

Checklist

• Reviewed by team member
• Reviewed by content team