fix: continue updating backends during shutdown

[grounded042]

What this PR does / why we need it:

Draft PR for fixing #13215. Will fill out more soon.

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] CVE Report (Scanner found CVE and adding report)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Documentation only

Which issue/s this PR fixes

fixes #13215

How Has This Been Tested?

I've tested it manually via make dev-env and following re-production steps in #13215

Checklist:

TODO

• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [ ] I've read the CONTRIBUTION guide
• [ ] I have added unit and/or e2e tests to cover my changes.
• [ ] All new and existing tests passed.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[k8s-ci-robot]

Hi @grounded042. Thanks for your PR.

I'm waiting for a kubernetes member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[netlify[bot]]

✅ Deploy Preview for kubernetes-ingress-nginx canceled.

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[strongjz]

Talked with @grounded042 during sync. he is going to add a go unit test and then we can review for 1.13 release.

[strongjz]

/triage accepted /ok-to-test /priority backlog /kind feature