Change address factory to reuse state before creating one
Add fallback to last incomplete spree order on current_order helper

* This allows users that not yet signed in with a guest_token assigned
  before to reuse their carts created from 2.2.

* There are no specs yet because the structure of sprees specs clutters
  the specification of `current_order` in various places. A strategy to
  cover the added behavior is till to be found.