order concurrency for 2-1-stable spree

* separate PR for 2-1-stable due for https://github.com/spree/spree/pull/4286

Fixes #4334

Conflicts:
	api/app/controllers/spree/api/checkouts_controller.rb
	api/app/controllers/spree/api/orders_controller.rb
	core/lib/spree/core/controller_helpers/order.rb
	frontend/spec/controllers/spree/current_order_tracking_spec.rb