Run sync task in its own thread

[medihack]

Closes #1124

Successful PR Checklist:

• [x] Tests
  • [ ] (not applicable?)
• [ ] Documentation
  • [ ] (not applicable?)

PR label(s):

• [ ] https://github.com/procrastinate-org/procrastinate/labels/PR%20type%3A%20breaking%20%F0%9F%92%A5
• [ ] https://github.com/procrastinate-org/procrastinate/labels/PR%20type%3A%20feature%20%E2%AD%90%EF%B8%8F
• [x] https://github.com/procrastinate-org/procrastinate/labels/PR%20type%3A%20bugfix%20%F0%9F%95%B5%EF%B8%8F
• [ ] https://github.com/procrastinate-org/procrastinate/labels/PR%20type%3A%20miscellaneous%20%F0%9F%91%BE
• [ ] https://github.com/procrastinate-org/procrastinate/labels/PR%20type%3A%20dependencies%20%F0%9F%A4%96
• [ ] https://github.com/procrastinate-org/procrastinate/labels/PR%20type%3A%20documentation%20%F0%9F%93%9A

As discussed in #1156, this may not be a breaking change but rather a fix for an issue, because synchronous tasks in concurrency mode currently don't run in parallel but consecutively.

[github-actions[bot]]

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File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
procrastinate
utils.py
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[ewjoachim]

(you can change the base of a PR, you don't have to recreate one. Use the "Edit" button to the right of the title)