Change the fields of a customer that the body states

The customer is written to the accounting platform first. With an `external_id` that a customer already carries, that customer is updated and the answer is 200; otherwise a customer is created and the answer is 201. An update replaces what the body states and keeps a choice from the platform's own lists (`vat_zone_ref`, `payment_terms_ref`, `layout_ref`, `customer_group_ref`) that the body leaves out. One create runs at a time under an `external_id`: a second while the first is still running is answered 409 `CREATE_IN_PROGRESS` with `Retry-After`, and the same request after that interval answers the customer the first created. A create under an `external_id` is safe to send again after a 502: where the first request made the customer, the same request, with the same body and within the hour, is answered that customer. A create with no `external_id` sent again makes a second customer. Where the platform made the customer and the create did not finish, the answer is 502 `CREATE_UNFINISHED` with the customer's `record_id`: the customer is in the catalog, and the same request again updates it.