Prevent deletion of resources created on behalf of the operator.

[sdudoladov]

We've had cases when users manually deleted pods/statefulsets/etc of a PG cluster only to found out them to be re-created later. This leads to users' confusion and extra debugging overhead for maintenance teams. We should experiment with k8s finalizers available since k8s v1.11 to prevent such deletions. Things to watch out are (a) properly handling the deletion timestamp (is set on the first delete request ), (b) effectively informing users about why their deletions of operator managed resources do not succeed and (c) enabling members of admin teams manually delete misbehaving resources.

A more sophisticated option is to use k8s admission webhooks.

[FxKu]

Closing this tickets since finalizers support has been added.