BulkImports: Add fields to track concurrent work

What does this MR do?

Add a way to track the status of each BulkImport's pipeline individually. The end goal is to enable BulkImports to run in smaller and more concurrent jobs. The pipeline status/progress will be tracked on the bulk_import_trackers table, using BulkImports::Tracker model.

More information on:

• #323382 (closed)
• &5544

Follow up:

• !56242 (merged) - Usage of the new fields to track the pipeline work status
• !55928 (merged) - Remove duplication on #after_run methods
• #323384 (closed) - Run each pipeline on its own background job

Database migrations

VERSION=20210303193544

== 20210303193544 AddConcurrentFieldsToBulkImportsTrackers: migrating =========
-- add_column(:bulk_import_trackers, :jid, :text)
   -> 0.0029s
-- add_column(:bulk_import_trackers, :stage, :smallint, {:default=>0, :null=>false})
   -> 0.0018s
-- add_column(:bulk_import_trackers, :status, :smallint, {:default=>0, :null=>false})
   -> 0.0012s
== 20210303193544 AddConcurrentFieldsToBulkImportsTrackers: migrated (0.0061s)

== 20210303193544 AddConcurrentFieldsToBulkImportsTrackers: reverting =========
-- remove_column(:bulk_import_trackers, :jid, :text)
   -> 0.0027s
-- remove_column(:bulk_import_trackers, :stage, :smallint)
   -> 0.0013s
-- remove_column(:bulk_import_trackers, :status, :smallint)
   -> 0.0009s
== 20210303193544 AddConcurrentFieldsToBulkImportsTrackers: reverted (0.0050s)

VERSION= 20210303193605

== 20210303193605 AddTextLimitsToBulkImportsTrackersJidAndPipelineName: migrating
-- transaction_open?()
   -> 0.0000s
-- current_schema()
   -> 0.0002s
-- execute("ALTER TABLE bulk_import_trackers\nADD CONSTRAINT check_603f91cb06\nCHECK ( char_length(jid) <= 255 )\nNOT VALID;\n")
   -> 0.0023s
-- current_schema()
   -> 0.0003s
-- execute("SET statement_timeout TO 0")
   -> 0.0008s
-- execute("ALTER TABLE bulk_import_trackers VALIDATE CONSTRAINT check_603f91cb06;")
   -> 0.0015s
-- execute("RESET ALL")
   -> 0.0009s
== 20210303193605 AddTextLimitsToBulkImportsTrackersJidAndPipelineName: migrated (0.0171s)

== 20210303193605 AddTextLimitsToBulkImportsTrackersJidAndPipelineName: reverting
-- execute("ALTER TABLE bulk_import_trackers\nDROP CONSTRAINT IF EXISTS check_603f91cb06\n")
   -> 0.0008s
== 20210303193605 AddTextLimitsToBulkImportsTrackersJidAndPipelineName: reverted (0.0061s)

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