Background migration to move requirement to work items

What does this MR do and why?

Background migration to migrate requirement objects into work items(issues) of requirement type.

This is part of a plan to deprecate old Requirement objects. For more information check: #323779 (closed)

This migration creates new issues directly on DB which means we have to generate and store latest issue iids in order to keep creation of new issues valid.

related to #342496 (closed)

Scheduling queries

There are only ~12500 records on the requirements table. I am using a 30 batch size for scheduling which should produce ~420 jobs. These should run for each batch scheduled.

 SELECT "requirements"."id" FROM "requirements" WHERE "requirements"."issue_id" IS NULL ORDER BY "requirements"."id" ASC LIMIT 1

https://explain.depesz.com/s/Fg9v#html

SELECT "requirements"."id" FROM "requirements" WHERE "requirements"."issue_id" IS NULL AND "requirements"."id" >= 1 ORDER BY "requirements"."id" ASC LIMIT 1 OFFSET 30

https://explain.depesz.com/s/Kh5v

SELECT MIN("requirements"."id"), MAX("requirements"."id") FROM "requirements" WHERE "requirements"."issue_id" IS NULL AND "requirements"."id" >= 1 AND "requirements"."id" < 38

https://explain.depesz.com/s/sHTw#html

Execution queries

All the queries below run inside a transaction for each requirement record:

  INSERT INTO issues (iid, issue_type, title, description, state_id, author_id, project_id, created_at, updated_at)
                  VALUES
                  (
                    COALESCE((SELECT MAX(iid) FROM issues WHERE project_id = 278964), 0) + 1,
                    3,
                    'A requirement work item',
                    NULL,
                    1,
                    426128,
                    278964,
                    '2021-10-18 18:50:12',
                    '2021-10-18 18:50:12'
                  ) RETURNING id, iid, project_id

https://explain.depesz.com/s/2fLO
duration: 55ms on database lab

UPDATE "requirements" SET "updated_at" = '2021-10-18 18:50:12', "issue_id" = 172 WHERE "requirements"."id" = 221

https://explain.depesz.com/s/RUvU7

UPDATE "internal_ids" SET "last_value" = GREATEST("internal_ids"."last_value", 1) WHERE "internal_ids"."project_id" = 278964 AND "internal_ids"."usage" = 0 RETURNING "last_value"

https://explain.depesz.com/s/HB9L

SELECT MAX("issues"."iid") FROM "issues" WHERE "issues"."project_id" = 278964

https://explain.depesz.com/s/uyAm

INSERT INTO "internal_ids" ("project_id","namespace_id","usage","last_value") VALUES (278964, NULL, 0, 1) ON CONFLICT  DO NOTHING RETURNING "id"

https://explain.depesz.com/s/wshG This one only runs if the update on internal id does not return a result