[Spike] Delivery requests: request processing once an item is checked in

According to UICHKIN-113, delivery request that seats on top of the queue for an item has be automatically processed in a way that

• the item is checked out,
• the request become closed as filled
• delivery staff slip is printed if configured

There is a need to design the solution for the above and it has to address the following points:

• understand how request processing is done in general (request priorities, queues, existing request processors and request states, request API etc)
• examine check-in/check-out functionality in details and point to its part(s) that can be useful for the solution
• prepare a design

POC to be created to confirm the approach.