Checkout Block: Mitigate and test for concurrency issues when reserving stock

[mikejolley]

Introduced in #1425, the API creates draft orders during the checkout flow which also help reserve stock during checkout for a customer.

Core team have been working on a similar system for putting holds on stock which attempts to mitigate concurrency issues (too much stock being reserved if 2 or more requests come in simultaneously).

• https://github.com/woocommerce/woocommerce/pull/25172/files#diff-27a48ce67fa604181c90b4bb464164acR306
• Add support for '_held_for_checkout` records to prevent race conditions. [part 2] woocommerce#25172

Since there is benefit to rolling out changes in blocks first, we should (with @woocommerce/proton and @vedanshujain) ensure the solution in blocks is flexible enough to be the solution used by core too, and ensure we have a testing tool to confirm this works.

TODO:

• Review current stock reservation table solution, align with core
• Add concurrency testing tool
• Implement protection against concurrency issues