Pod Rolling update

introduce Pod events channel;
add parsing of the MaintenanceWindows section;
skip deleting Etcd key on cluster delete;
use external etcd host;
watch for tpr/pods in the namespace of the operator pod only;

manifests/platform-credentials.yaml

@@ -2,6 +2,7 @@ apiVersion: "zalando.org/v1"
 kind: PlatformCredentialsSet
 metadata:
   name: postgresql-operator
+  namespace: acid
 spec:
   application: postgresql-operator
   tokens: