All queue ShutDown() calls should be able to be invoked multiple times.
```
Observed a panic: "close of closed channel" (close of closed channel)
/go/src/github.com/openshift/cluster-version-operator/vendor/k8s.io/apimachinery/pkg/util/runtime/runtime.go:76
/go/src/github.com/openshift/cluster-version-operator/vendor/k8s.io/apimachinery/pkg/util/runtime/runtime.go:65
/go/src/github.com/openshift/cluster-version-operator/vendor/k8s.io/apimachinery/pkg/util/runtime/runtime.go:51
/usr/local/go/src/runtime/asm_amd64.s:573
/usr/local/go/src/runtime/panic.go:502
/usr/local/go/src/runtime/chan.go:333
/go/src/github.com/openshift/cluster-version-operator/vendor/k8s.io/client-go/util/workqueue/delaying_queue.go:137
```
Use sync.Once to guarantee a single close.
Kubernetes-commit: d2f7eb5235a93556261c8947e7a87342aeeaee2b