Ignore updates of ingresses with invalid class

[ArchangelSDY]

What this PR does / why we need it:

Ignore updates of ingresses with invalid class. This avoids unnecessary nginx reload which can be really expensive in websocket scenario.

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #

Special notes for your reviewer:

When I run unit tests, the race detector reports some data races regarding k8sStore.backendConfig. I added a RWMutex to protect against simultaneous read/write of that. In addition, I also changed k8sStore member functions to receive a k8sStore pointer rather than a value to avoid copy, which implies read of backendConfig.

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[ArchangelSDY]

CLA signed.

[aledbf]

/lgtm
/approve

[aledbf]

@ArchangelSDY thanks!

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aledbf, ArchangelSDY

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