Skips Load Balancer from Feature::FlipperRecord

What does this MR do and why?

When feature flags get checked outside of the rails executor, a connection is checked out from the rails connection pool but isn't ever checked back in as part of the executor cleanup.

Feature::FlipperRecord does not use our load balancer, but the changes made in https://gitlab.com/gitlab-org/gitlab/blob/master/lib/gitlab/database/load_balancing/setup.rb#L53 recently was making it to go through the LB connectionProxy.

This MR makes FF to properly bypass the LB by extending SkipLoadBalancer module.

Issue: Investigate dead connections in gitlab rails co... (#588806)

References

How to set up and validate locally

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