Add BulkImports::Tracker

What does this MR do?

Related to:

• #250281 (closed)
• !46710 (merged)

This new table is required to support the Group Migration feature. When importing data from another GitLab instance a lot of HTTP/GraphQL requests are done to the source instance to fetch data. Depending on the size of the source, many of these requests will be paginated, BulkImports::Tracker will be used to keep track of what pages were already processed and if the relation still have pages to be processed.

Why the next_page field is not an integer?

The next_page is a text field and not integer is because some of the requests are done over GraphQL, which uses a cursor based pagination, which is a string.

Example

To fetch a group Epics, for instance, we would save something like:

example description	relation	has_next_page	next_page
with graphql	epics	true	eyJpZCI6IjEzNzE5NCJ9
with rest	labels	true	2
when there's no more pages to fetch	milestones	false	null

Migration

• CreateBulkImportTracker

$ rails db:migrate:up VERSION=20201104204739
== 20201104204739 CreateBulkImportTrackers: migrating =========================
-- create_table(:bulk_import_trackers)
   -> 0.0087s
-- transaction_open?()
   -> 0.0000s
-- current_schema()
   -> 0.0002s
-- execute("ALTER TABLE bulk_import_trackers\nADD CONSTRAINT check_2d45cae629\nCHECK ( char_length(relation) <= 255 )\nNOT VALID;\n")
   -> 0.0004s
-- current_schema()
   -> 0.0002s
-- execute("SET statement_timeout TO 0")
   -> 0.0002s
-- execute("ALTER TABLE bulk_import_trackers VALIDATE CONSTRAINT check_2d45cae629;")
   -> 0.0004s
-- execute("RESET ALL")
   -> 0.0001s
-- transaction_open?()
   -> 0.0000s
-- current_schema()
   -> 0.0001s
-- execute("ALTER TABLE bulk_import_trackers\nADD CONSTRAINT check_40aeaa600b\nCHECK ( char_length(next_page) <= 255 )\nNOT VALID;\n")
   -> 0.0003s
-- current_schema()
   -> 0.0001s
-- execute("ALTER TABLE bulk_import_trackers VALIDATE CONSTRAINT check_40aeaa600b;")
   -> 0.0004s
== 20201104204739 CreateBulkImportTrackers: migrated (0.0187s) ================

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• 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