[Tech]: Delete order book optimization

[wer1st]

Problem statement

By benchmarks we found that order-book deleting takes too much time in worst case. Necessary to optimize that process.

Description

It is needed to implement new approach for order-book deleting.
Now we can delete only empty order-book in states: OnlyCancel or Stop.

Dev changes:

• Don't cancel any orders
• Check that order-book is empty
• Check that order-book has states OnlyCancel or Stop
• Delete order-book if all conditions are met.

Delete order-book from the user perspective:

1. Announce the delisting of pair from order book liquidity source.
2. Change order-book status to OnlyCancel or Stop.
3. Wait untill users cancel their limit orders or they expire. Order-book must be empty.
4. Call order-book::delete_orderbook() extrinsic.

Definition of Done

• Order-book is not deleted if it has any statuses other thanOnlyCancel or Stop.
• Order-book is not deleted if it has at least one limit-order.
• Deleting of order-book doesn't cancell any orders.
• Order-book is deleted by delete_orderbook extinsic if all conditions are met.
• Update unit tests.
• Update benchmarks.
• Update autotests if it necessary.

Requirements

No response

[bragov4ik]

A suggestion: instead of disallowing removal of order books that contain any orders, we can allow cancellation of up to $N$ orders, where $N$ is small enough so that the removal is as short as needed.

[bragov4ik]

we can allow cancellation of up to $N$ orders
After a discussion we concluded that we won't implement it this way because

• It can be ambiguous from user perspective (sounds like an arbitrary and weird restriction)
• There is no easy indicator of order book having $N$ orders, in contrast to being empty
• The implementation would be additional code that can introduce additional bugs, weaknesses, etc.
• There is no clear motivation to allow quick(-er) removal of the order book

[ra9mls]

https://app.qase.io/case/PSS-159