Backport the 1207 fix to persistent-2.11.0

[parsonsmatt]

Before submitting your PR, check that you've:

• Documented new APIs with Haddock markup
• Added @since declarations to the Haddock

After submitting your PR:

• Update the Changelog.md file with a link to your PR
• Bumped the version number if there isn't an (unreleased) on the Changelog
• Check that CI passes (or if it fails, for reasons unrelated to your change, like CI timeouts)

[codygman]

Alright using:

  - git: https://github.com/yesodweb/persistent
    commit: f496bbdd74f8b26be1d09dd0bfcc697bd7d8d48b
    subdirs:
     - persistent
     - persistent-mongoDB
     - persistent-postgresql
     - persistent-template

It passed:

• 1 2 times from clean slate in CI
• 10 times in ghcid

I'll keep running it in CI which starts from a clean slate and see what happens.

[parsonsmatt]

The problem occurred when running against the single threaded runtime as well as the threaded runtime, and only was alleviated by having more and more threads available (eg -N2 had more errors than -N4 etc). Not sure if ghcid would exacerbate or ameliorate the issue.

Here's hoping for those good CI passes 😄