Improve the perfomance of finding cancelable pipelines for users

What does this MR do?

Improves the performance of the query used to identify cancelable pipelines for a given user.

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/301222

Screenshots (strongly suggested)

• Query and execution plans before adding the index: https://console.postgres.ai/shared/33a94e3c-9ae4-4c95-a851-f99d5732dc53
• Adding the index: https://gitlab.slack.com/archives/CLJMDRD8C/p1615470333112700 - The query has been executed. Duration: 2402.394 s (estimated* for prod: 200.456...2402.394 s)
• Query and execution plans after adding the index: https://console.postgres.ai/shared/4f651cd1-7348-43ac-bc21-d0bd8151bd90

Up

VERSION=20210311093723 bin/rails db:migrate:up
== 20210311093723 AddPartialIndexOnCiPipelinesByCancelableStatusAndUsers: migrating
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:ci_pipelines, [:user_id, :id], {:where=>"((status)::text = ANY (\n  ARRAY[\n    ('running'::character varying)::text,\n    ('waiting_for_resource'::character varying)::text,\n    ('preparing'::character varying)::text,\n    ('pending'::character varying)::text,\n    ('created'::character varying)::text,\n    ('scheduled'::character varying)::text\n  ]\n))\n", :name=>"index_ci_pipelines_on_user_id_and_id_and_cancelable_status", :algorithm=>:concurrently})
   -> 0.0094s
-- execute("SET statement_timeout TO 0")
   -> 0.0010s
-- add_index(:ci_pipelines, [:user_id, :id], {:where=>"((status)::text = ANY (\n  ARRAY[\n    ('running'::character varying)::text,\n    ('waiting_for_resource'::character varying)::text,\n    ('preparing'::character varying)::text,\n    ('pending'::character varying)::text,\n    ('created'::character varying)::text,\n    ('scheduled'::character varying)::text\n  ]\n))\n", :name=>"index_ci_pipelines_on_user_id_and_id_and_cancelable_status", :algorithm=>:concurrently})
   -> 0.0094s
-- execute("RESET ALL")
   -> 0.0009s
== 20210311093723 AddPartialIndexOnCiPipelinesByCancelableStatusAndUsers: migrated (0.0224s)

Down

 VERSION=20210311093723 bin/rails db:migrate:down
== 20210311093723 AddPartialIndexOnCiPipelinesByCancelableStatusAndUsers: reverting
-- transaction_open?()
   -> 0.0000s
-- indexes(:ci_pipelines)
   -> 0.0135s
-- execute("SET statement_timeout TO 0")
   -> 0.0006s
-- remove_index(:ci_pipelines, {:algorithm=>:concurrently, :name=>"index_ci_pipelines_on_user_id_and_id_and_cancelable_status"})
   -> 0.0041s
-- execute("RESET ALL")
   -> 0.0008s
== 20210311093723 AddPartialIndexOnCiPipelinesByCancelableStatusAndUsers: reverted (0.0207s)

Does this MR meet the acceptance criteria?

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