Delivery state machine

Delivery state machine defines the states and transitions of a delivery.

A single order may have multiple deliveries, and therefore may have multiple delivery states (one status for each delivery).

The implementation project may decide to customize delivery state machine by adding or removing states, or modifing the various conditions and transitions among them.

The default delivery state machine has only four states:

1. Init: Initial state.
2. Delivered: Delivery is successful.
3. Failed: Delivery failed.
4. Cancelled: Delivery is cancelled.

When a delivery goes to "Delivered" state, order will be automatically put into "Processing" state.

If all deliveries are in "Delivered" and all payments are "Paid", order is automatically put into "Completed" order state.

If at least one delivery fails, the order is put into "Attention" state.