Prepare ci_stages for bigint conversion

What does this MR do?

Prepare conversion of ci_stages.id to bigint.

Related to #327622 (closed)

Migration Output

Up

== 20210608072312 InitializeConversionOfCiStagesToBigint: migrating ===========
-- table_exists?(:ci_stages)
   -> 0.0017s
-- column_exists?(:ci_stages, :id)
   -> 0.0031s
-- column_exists?(:ci_stages, :id)
   -> 0.0012s
-- columns(:ci_stages)
   -> 0.0013s
-- add_column(:ci_stages, "id_convert_to_bigint", :bigint, {:default=>0, :null=>false})
   -> 0.0071s
== 20210608072312 InitializeConversionOfCiStagesToBigint: migrated (0.0417s) ==

== 20210608072346 BackfillCiStagesForBigintConversion: migrating ==============
-- table_exists?(:ci_stages)
   -> 0.0012s
-- column_exists?(:ci_stages, :id)
   -> 0.0013s
-- column_exists?(:ci_stages, :id)
   -> 0.0014s
-- column_exists?(:ci_stages, "id_convert_to_bigint")
   -> 0.0013s
unknown OID 28: failed to recognize type of 'relfrozenxid'. It will be treated as String.
unknown OID 1034: failed to recognize type of 'relacl'. It will be treated as String.
unknown OID 194: failed to recognize type of 'relpartbound'. It will be treated as String.
== 20210608072346 BackfillCiStagesForBigintConversion: migrated (0.0591s) =====

Down

== 20210608072346 BackfillCiStagesForBigintConversion: reverting ==============
-- execute("DELETE FROM batched_background_migrations WHERE job_class_name = 'CopyColumnUsingBackgroundMigrationJob' AND table_name = 'ci_stages' AND column_name = 'id' AND job_arguments = '[[\"id\"],[\"id_convert_to_bigint\"]]'")
   -> 0.0029s
== 20210608072346 BackfillCiStagesForBigintConversion: reverted (0.0031s) =====

== 20210608072312 InitializeConversionOfCiStagesToBigint: reverting ===========
-- remove_column(:ci_stages, "id_convert_to_bigint")
   -> 0.0025s
== 20210608072312 InitializeConversionOfCiStagesToBigint: reverted (0.0073s) ==

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