Merge branch 'fix/gb/stage-id-reference-background-migration' into backstage/gb/migrate-stages-statuses

* fix/gb/stage-id-reference-background-migration: (22 commits)
  Reduce a delay between stage_id scheduled migrations
  Improve exception description in bg migrations
  Do not override original AR5 batching interface
  Sanitize id value passed to async background migration
  Improve code examples in background migrations docs
  Add description to exception in bg migrations worker
  Do not compare float with integer in migrations specs
  Improve readability of build stage id migration query
  Use integers to schedule delayed background migrations
  Test if argument passed to a migration is present
  Make `inline` a default sidekiq testing processing again
  Improve specs for background stage_id ref migration
  Perform stage_id ref backgound migration in bulks
  Remove unused background migrations matcher
  Use ActiveRecord 5 batching to schedule bg migration
  Make it possible to schedule bg migrations in bulk
  Add specs for delayed stage_id background migrations
  Schedule background migration only when it is needed
  Find builds that require a migration in batches
  Update `db/schema.rb` with a new schema version
  ...