Blueprint+Design: Database bloat analysis + pg_repack

This contains an analysis of the database bloat. It shows that 95% of the overall database bloat are caused by regular indexes (not by table bloat, not by bloat of primary key indexes).

Epic: gitlab-com/gl-infra&29 (closed)

Blueprint: https://ab-5994-pgbloat.about-src.gitlab.com/handbook/engineering/infrastructure/blueprint/201901-postgres-bloat/index.html

Design: https://ab-5994-pgbloat.about-src.gitlab.com/handbook/engineering/infrastructure/design/201902_PostgreSQL_Bloat.html

Closes https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/5994.