Targeted query optimization for CI queueing

This is a follow-up to gitlab-com/gl-infra/production#4156 (comment 546835041).

Query

• Plan: https://explain.depesz.com/s/8NUQ
• Timings outside of the incident: This is taking 9s on a luke-warm cache and still 3.5s on a hot cache (replica).

SELECT
    "ci_builds"."id"
FROM
    "ci_builds"
    INNER JOIN "projects" ON "projects"."id" = "ci_builds"."project_id"
    LEFT JOIN project_features ON ci_builds.project_id = project_features.project_id
    LEFT JOIN (
        SELECT
            "ci_builds"."project_id",
            count(*) AS running_builds
        FROM
            "ci_builds"
        WHERE
            "ci_builds"."type" = 'Ci::Build'
            AND ("ci_builds"."status" IN ('running'))
            AND "ci_builds"."runner_id" IN (
                SELECT
                    "ci_runners"."id"
                FROM
                    "ci_runners"
                WHERE
                    "ci_runners"."runner_type" = 1)
            GROUP BY
                "ci_builds"."project_id") AS project_builds ON ci_builds.project_id = project_builds.project_id
WHERE ("ci_builds"."status" IN ('pending'))
AND "ci_builds"."runner_id" IS NULL
AND "projects"."shared_runners_enabled" = TRUE
AND "projects"."pending_delete" = FALSE
AND (project_features.builds_access_level IS NULL
    OR project_features.builds_access_level > 0)
AND "ci_builds"."type" = 'Ci::Build'
AND ("projects"."visibility_level" = 20
    OR (EXISTS ( WITH RECURSIVE "base_and_ancestors" AS (
(
                    SELECT
                        "namespaces".*
                    FROM
                        "namespaces"
                    WHERE (namespaces.id = projects.namespace_id))
            UNION (
                SELECT
                    "namespaces".*
                FROM
                    "namespaces",
                    "base_and_ancestors"
                WHERE
                    "namespaces"."id" = "base_and_ancestors"."parent_id"))
            SELECT DISTINCT
                1
            FROM
                "base_and_ancestors" AS "namespaces"
            LEFT JOIN namespace_statistics ON namespace_statistics.namespace_id = namespaces.id
        WHERE
            "namespaces"."parent_id" IS NULL
            AND (COALESCE(namespaces.shared_runners_minutes_limit, 400, 0) = 0
                OR COALESCE(namespace_statistics.shared_runners_seconds, 0) < COALESCE((namespaces.shared_runners_minutes_limit + COALESCE(namespaces.extra_shared_runners_minutes_limit, 0)), (400 + COALESCE(namespaces.extra_shared_runners_minutes_limit, 0)), 0) * 60))))
AND (NOT EXISTS (
        SELECT
            1
        FROM
            "taggings"
        WHERE
            "taggings"."taggable_type" = 'CommitStatus'
            AND "taggings"."context" = 'tags'
            AND (taggable_id = ci_builds.id)
            AND "taggings"."tag_id" NOT IN (4060, 14174)))
ORDER BY
    COALESCE(project_builds.running_builds, 0) ASC, ci_builds.id ASC