Remove cross joins for ProjectMonthlyUsage.for_namespace_monthly_usage

Ref: #343301 (closed)

What does this MR do and why?

This MR removes a cross joins query for Ci::Minutes::ProjectMonthlyUsage.for_namespace_monthly_usage.

This change is behind a feature flag : ci_decompose_for_namespace_monthly_usage_query.

Database review

Old SQL Query

SELECT
  ci_project_monthly_usages.*
FROM
  ci_project_monthly_usages
WHERE
    ci_project_monthly_usages.date = '2021-05-01'
  AND ci_project_monthly_usages.project_id IN (
        SELECT
    projects.id
  FROM
    projects
  WHERE
    projects.namespace_id = 278964)

Run the following commands to backfill our new ci mirror tables which will happen in !76327 (merged).

exec INSERT INTO ci_project_mirrors (project_id, namespace_id)
SELECT "projects"."id", "projects"."namespace_id" FROM "projects"
ON CONFLICT (project_id) DO NOTHING;

exec INSERT INTO ci_namespace_mirrors (namespace_id, traversal_ids)
SELECT "namespaces"."id", "namespaces"."traversal_ids" FROM "namespaces"
ON CONFLICT (namespace_id) DO NOTHING;

exec ANALYZE ci_project_mirrors;
exec ANALYZE ci_namespace_mirrors;

New SQL Query

SELECT
    ci_project_monthly_usages.*
FROM
    ci_project_monthly_usages
WHERE
    ci_project_monthly_usages.date = '2021-05-01'
    AND ci_project_monthly_usages.project_id IN (
        SELECT
            ci_project_mirrors.project_id
        FROM
            ci_project_mirrors
        WHERE
            ci_project_mirrors.namespace_id = 278964);

Other SQL Query considered (not retained)

SELECT
    ci_project_monthly_usages.*
FROM
    ci_project_monthly_usages
WHERE
    ci_project_monthly_usages.date = '2021-05-01'
    AND ci_project_monthly_usages.project_id IN (98024, 116212, 140724, 143237, 145205, 150440, 227582, 250324, 250833, 278964, 280425, 375711, 387896, 413007, 430285, 443787, 444821, 455030, 480929, 554859, 593728, 629054, 629060, 684698, 730448, 734943, 747741, 766015, 818896, 876090, 887372, 928825, 931715, 998792, 1075790, 1120019, 1209837, 1265999, 1329047, 1379171, 1441932, 1470839, 1507906, 1533158, 1777822, 1794617, 1911766, 1990920, 2009901, 2127625, 2317465, 2337675, 2347063, 2383700, 2651596, 2670515, 2694799, 2725567, 2890326, 2953390, 3010986, 3010998, 3094319, 3101096, 3305972, 3362933, 3466815, 3588247, 3605985, 3631141, 3651684, 3662568, 3662668, 3674569, 3698388, 3871132, 3871556, 3885956, 3885980, 3933206, 3933372, 3991945)

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.