ENH: implementation of an option to add 'queue_stop' instructions to the plan queue

[dmgav]

The code that implements an option to add queue_stop instructions to the plan queue. Once the instruction is encountered during plan execution, the queue execution is stopped. Execution may be resumed by sending queue_start request to the manager. The instructions are treated similarly to plans: they can be added to the queue, removed from the queue or moved within the queue. Instructions are assigned plan_uid and can be accessed by position and UID (the same way as plans).

Currently an instruction can be added to the queue by sending

"instruction": {"action": "queue_stop"}

instead of "plan": {...} as an argument of queue_plan_add. The option of adding instructions is not yet implemented in CLI tool.

The option of adding instructions to the queue makes current naming (e.g. queue_plan_add, queue_plan_remove, plan_uid) misleading. It may be a good idea to rename the API that are used to request operations on the queue and data fields reporting the state of the queue to use the word item instead of plan (e.g. queue_item_add, queue_item_remove, item_uid).

[mrakitin]

It's a good addition, I see no issues with it.

[dmgav]

Thanks @mrakitin. I'll start working on renaming things.