Use namespace list from namespaceWatcher.NamespaceStore instead of retrieving it (#132)

This fixes performance issues when many secrets need to be replicated to
other namespaces via replicate-to.

The performance issue would be acceptable, when it would fix itself after
some time after the initial replication after startup. However, with the
current solution, the liveness probe never gets green and keep causing
replicator restarts in our clusters.

Co-authored-by: Martin Helmich <m.helmich@mittwald.de>

Author: codablock

replicate/common/generic-replicator.go

@@ -269,9 +269,12 @@ func (r *GenericReplicator) ResourceAdded(obj interface{}) {
 	if namespacePatterns, ok := annotations[ReplicateTo]; ok {
 		r.ReplicateToList[sourceKey] = struct{}{}
 
-		if list, err := r.Client.CoreV1().Namespaces().List(ctx, metav1.ListOptions{}); err != nil {
-			logger.WithError(err).Error("failed to list namespaces")
-		} else if err := r.replicateResourceToMatchingNamespaces(obj, namespacePatterns, list.Items); err != nil {
+		namespacesFromStore := namespaceWatcher.NamespaceStore.List()
+		namespaces := make([]v1.Namespace, len(namespacesFromStore))
+		for i, ns := range namespacesFromStore {
+			namespaces[i] = *ns.(*v1.Namespace)
+		}
+		if err := r.replicateResourceToMatchingNamespaces(obj, namespacePatterns, namespaces); err != nil {
 			logger.WithError(err).Errorf("could not replicate object to other namespaces")
 		}
 	} else {