Re-introduce fix for Hydra directory sync with multiple process

[SeanNaren]

What does this PR do?

Re-introduce fix from #5629 that was left out of the accelerator refactor highlighted by @mees! Also there seems there was an edge case that caused some issues with @mees setup, and he suggested a slight modification to the fix.

Have confirmed locally that this fixes the issue.

Before submitting

• Was this discussed/approved via a GitHub issue? (not for typos and docs)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you make sure to update the documentation with your changes? (if necessary)
• Did you write any new necessary tests? (not for typos and docs)
• Did you verify new and existing tests pass locally with your changes?
• Did you update the CHANGELOG? (not for typos, docs, test updates, or internal minor changes/refactorings)

PR review

Anyone in the community is free to review the PR once the tests have passed.
Before you start reviewing make sure you have read Review guidelines. In short, see the following bullet-list:

• Is this pull request ready for review? (if not, please submit in draft mode)
• Check that all items from Before submitting are resolved
• Make sure the title is self-explanatory and the description concisely explains the PR
• Add labels and milestones (and optionally projects) to the PR so it can be classified
• Check that target branch and milestone match!

Did you have fun?

Make sure you had fun coding 🙃

[codecov]

Codecov Report

Merging #5993 (4593672) into master (6a9cec4) will decrease coverage by 0%.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #5993   +/-   ##
======================================
- Coverage      93%     93%   -0%     
======================================
  Files         160     160           
  Lines       11344   11344           
======================================
- Hits        10554   10525   -29     
- Misses        790     819   +29     

[SeanNaren]

I'll try get a test for this in place, I had difficulty the first time but this time round might be doable...

[tchaton]

LGTM !