Prepare indexes on events for bigint column conversions

Add migration to create indexes asynchronously for the `events` table.
Creating the indexes asynchronously will preventing the column swap
migration from running for an excessive length of time.

Changelog: other

db/post_migrate/20210817024335_prepare_indexes_for_events_bigint_conversion.rb

+# frozen_string_literal: true
+
+class PrepareIndexesForEventsBigintConversion < ActiveRecord::Migration[6.1]
+  include Gitlab::Database::MigrationHelpers
+
+  TABLE_NAME = 'events'
+
+  def up
+    prepare_async_index TABLE_NAME, :id_convert_to_bigint, unique: true,
+      name: :index_events_on_id_convert_to_bigint
+
+    prepare_async_index TABLE_NAME, [:project_id, :id_convert_to_bigint],
+      name: :index_events_on_project_id_and_id_convert_to_bigint
+
+    prepare_async_index TABLE_NAME, [:project_id, :id_convert_to_bigint], order: { id_convert_to_bigint: :desc },
+      where: 'action = 7', name: :index_events_on_project_id_and_id_bigint_desc_on_merged_action
+  end
+
+  def down
+    unprepare_async_index_by_name TABLE_NAME, :index_events_on_id_convert_to_bigint
+    unprepare_async_index_by_name TABLE_NAME, :index_events_on_project_id_and_id_convert_to_bigint
+    unprepare_async_index_by_name TABLE_NAME, :index_events_on_project_id_and_id_bigint_desc_on_merged_action
+  end
+end

db/schema_migrations/20210817024335

+360bb1c16c93d7a6564ed70fa2dea4212e1fd00d101cfdc9017b54f67eae797d
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