Async namespace_id based index for issue lists

What does this MR do and why?

This creates the indexes asynchronously for GitLab.com. This index will be used to replace existing project_id based indexes.

This index improves the performance of our group-level work items lists and also prepares us when we switch the project-level list to use namespace_id.

Local testing of bundle exec rails gitlab:db:execute_async_index_operations:all

gitlabhq_development=# \d index_issues_on_namespace_id_created_at_id_state_id
Index "public.index_issues_on_namespace_id_created_at_id_state_id"
    Column    |            Type             | Key? |  Definition
--------------+-----------------------------+------+--------------
 namespace_id | bigint                      | yes  | namespace_id
 created_at   | timestamp without time zone | yes  | created_at
 id           | bigint                      | yes  | id
 state_id     | smallint                    | yes  | state_id
btree, for table "public.issues"

gitlabhq_development=# \d index_issues_on_work_item_type_id_namespace_id_created_at_state
Index "public.index_issues_on_work_item_type_id_namespace_id_created_at_state"
      Column       |            Type             | Key? |    Definition
-------------------+-----------------------------+------+-------------------
 work_item_type_id | bigint                      | yes  | work_item_type_id
 namespace_id      | bigint                      | yes  | namespace_id
 created_at        | timestamp without time zone | yes  | created_at
 state_id          | smallint                    | yes  | state_id
btree, for table "public.issues"

gitlabhq_development=# \d index_issues_on_namespace_id_relative_position_id_state_id
Index "public.index_issues_on_namespace_id_relative_position_id_state_id"
      Column       |   Type   | Key? |    Definition
-------------------+----------+------+-------------------
 namespace_id      | bigint   | yes  | namespace_id
 relative_position | integer  | yes  | relative_position
 id                | bigint   | yes  | id
 state_id          | smallint | yes  | state_id
btree, for table "public.issues"

gitlabhq_development=# \d index_open_issues_on_namespace_id_confidential_author_id_id
Index "public.index_open_issues_on_namespace_id_confidential_author_id_id"
    Column    |  Type   | Key? |  Definition
--------------+---------+------+--------------
 namespace_id | bigint  | yes  | namespace_id
 confidential | boolean | yes  | confidential
 author_id    | bigint  | yes  | author_id
 id           | bigint  | yes  | id
btree, for table "public.issues", predicate (state_id = 1)

gitlabhq_development=# \d index_closed_incidents_on_namespace_id_closed_at
Index "public.index_closed_incidents_on_namespace_id_closed_at"
    Column    |           Type           | Key? |  Definition
--------------+--------------------------+------+--------------
 namespace_id | bigint                   | yes  | namespace_id
 closed_at    | timestamp with time zone | yes  | closed_at
btree, for table "public.issues", predicate (state_id = 2 AND work_item_type_id = 2)

References

Related to #536350 (closed), #536353 (closed)

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