Migration for fixing missing traversal ids

What does this MR do?

The first background migration job to backfill namespaces.traversal_ids did not cover all namespaces. This is most likely due to a misunderstanding we had between the relationship of chatops and default feature flag enablement.

This MR schedules the same background migrations as from the first background migration. Namely Gitlab::BackgroundMigration::BackfillNamespaceTraversalIdsRoots and Gitlab::BackgroundMigration::BackfillNamespaceTraversalIdsChildren.

Database

• Index creation took only a few minutes (both indexes). The indexes will contain about 100K namespaces.
• Follow-up for removing the indexes: #333365 (closed)

UP:

== 20210611080951 FixMissingTraversalIds: migrating ===========================
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:namespaces, :id, {:where=>"parent_id IS NULL AND traversal_ids = '{}'", :name=>"tmp_index_namespaces_empty_traversal_ids_with_root_namespaces", :algorithm=>:concurrently})
   -> 0.0091s
-- add_index(:namespaces, :id, {:where=>"parent_id IS NULL AND traversal_ids = '{}'", :name=>"tmp_index_namespaces_empty_traversal_ids_with_root_namespaces", :algorithm=>:concurrently})
   -> 0.0113s
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:namespaces, :id, {:where=>"parent_id IS NOT NULL AND traversal_ids = '{}'", :name=>"tmp_index_namespaces_empty_traversal_ids_with_child_namespaces", :algorithm=>:concurrently})
   -> 0.0260s
-- add_index(:namespaces, :id, {:where=>"parent_id IS NOT NULL AND traversal_ids = '{}'", :name=>"tmp_index_namespaces_empty_traversal_ids_with_child_namespaces", :algorithm=>:concurrently})
   -> 0.0078s
-- Scheduled 1 BackfillNamespaceTraversalIdsRoots jobs with a maximum of 1000 records per batch and an interval of 120 seconds.

The migration is expected to take at least 120 seconds. Expect all jobs to have completed after 2021-06-11 08:21:25 UTC."
-- Scheduled 1 BackfillNamespaceTraversalIdsChildren jobs with a maximum of 1000 records per batch and an interval of 120 seconds.

The migration is expected to take at least 360 seconds. Expect all jobs to have completed after 2021-06-11 08:25:25 UTC."
== 20210611080951 FixMissingTraversalIds: migrated (0.1908s) ==================

DOWN:

== 20210611080951 FixMissingTraversalIds: reverting ===========================
-- transaction_open?()
   -> 0.0000s
-- indexes(:namespaces)
   -> 0.0117s
-- execute("SET statement_timeout TO 0")
   -> 0.0012s
-- remove_index(:namespaces, {:algorithm=>:concurrently, :name=>"tmp_index_namespaces_empty_traversal_ids_with_root_namespaces"})
   -> 0.0036s
-- execute("RESET ALL")
   -> 0.0006s
-- transaction_open?()
   -> 0.0000s
-- indexes(:namespaces)
   -> 0.0093s
-- remove_index(:namespaces, {:algorithm=>:concurrently, :name=>"tmp_index_namespaces_empty_traversal_ids_with_child_namespaces"})
   -> 0.0028s
== 20210611080951 FixMissingTraversalIds: reverted (0.0317s) ==================

Queries:

2 standard EachBatch queries are covered by index.

• Scheduling Roots query 1: https://explain.depesz.com/s/PA9U
• Scheduling Roots query 2: https://explain.depesz.com/s/ZhKU
• Scheduling Children query 1: https://explain.depesz.com/s/YlAy
• Scheduling Children query 2: https://explain.depesz.com/s/Aqka

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team