Sandbox for identity support

sandbox4integer.md

• default nil for backward compatibility

https://github.com/rails/rails/blob/690d2a49465cf18b93cf66b77967cb1236c881cb/activerecord/lib/active_record/connection_adapters/postgresql/schema_definitions.rb#L33-L37

• MySQL auto_increment(JA)

https://dev.mysql.com/doc/refman/5.6/ja/example-auto-increment.html

• integer_like_primary_key_type

https://github.com/rails/rails/blob/690d2a49465cf18b93cf66b77967cb1236c881cb/activerecord/lib/active_record/connection_adapters/mysql/schema_definitions.rb#L76-L79

• Extract the short-hand column methods into ColumnMethods

rails/rails#19030

• auto_increment / bigint

https://github.com/rails/rails/blob/d47357e2d3d1d5333780488daa77ad9c012c147e/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb#L10-L12

• primary_key?

https://github.com/rails/rails/blob/690d2a49465cf18b93cf66b77967cb1236c881cb/activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb#L51-L53

• integer_to_sql

https://github.com/rails/rails/blob/690d2a49465cf18b93cf66b77967cb1236c881cb/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb#L827-L836