Have TGB handler reset checkpoint properly on cleanUp().

[zac-nixon]

Issue

#4343

Description

The current check pointing logic returns old check point = new check point = "". This means, when clean up is invoked for a now existent service, the checkpoint doesn't get updated due to this check. This leads to the old checkpoint being persisted in the TGB, despite the target group being updated. In the users case, the endpoint composition didn't change therefore when the service backend was corrected our TGB handler didn't re-register the targets.

The fix is quite simple, instead of setting both checkpoints to empty, we return the oldCheckPoint as the actual checkpoint, and the new checkpoint as "". This was the checkpoint is properly updated and the handler will re-register the targets when the service is fixed.

Checklist

• Added tests that cover your change (if possible)
• Added/modified documentation as required (such as the README.md, or the docs directory)
• Manually tested
• Made sure the title of the PR is a good description that can go into the release notes

BONUS POINTS checklist: complete for good vibes and maybe prizes?! 🤯

• Backfilled missing tests for code in same general area 🎉
• Refactored something and made the world a better place 🌟

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: zac-nixon

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[shuqz]

/approved

[shuqz]

/lgtm

[k8s-ci-robot]

@zac-nixon: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-aws-load-balancer-controller-e2e-test	0a2b938	link	true	/test pull-aws-load-balancer-controller-e2e-test

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