Order generation subscriber results in duplicates or missing report items

Problem/Motivation

The event subscriber doesn't do any locking. I'm seeing multiple HTTP requests getting processed at the end of a purchase transaction. This results in order report items being generated in duplicate. This could also result in data loss, per the TODO in the code. Let's figure out a better way to generate this.

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet