Dynamic LB sync non-external backends only when necessary

[aledbf]

What this PR does / why we need it:

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Which issue/s this PR fixes

fixes #5418
fixes #4808

How Has This Been Tested?

Checklist:

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

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aledbf

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [aledbf]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[aledbf]

/retest

[aledbf]

/retest

[aledbf]

/retest

[aledbf]

/retest

[aledbf]

/assign @ElvinEfendi

[aledbf]

@ZxYuan are you ok if we tried to merge this PR?. Is #5418 with the required fixes to pass e2e tests.
Or you want to finish #5418?

[ZxYuan]

@ZxYuan are you ok if we tried to merge this PR?. Is #5418 with the required fixes to pass e2e tests.
Or you want to finish #5418?

lgtm

[ElvinEfendi]

@aledbf are you planning to write some tests?

[aledbf]

@aledbf are you planning to write some tests?

Can you help me with that?

[ZxYuan]

@aledbf are you planning to write some tests?

Can you help me with that?

So sorry that I've been tied up recently.