[improve][broker]Ensure namespace deletion doesn't fail

[eolivelli]

Problem

Deleting a namespace sometimes fails and leaves znodes around, especially the "bundle owner" nodes /namespace/NAMESPACE/xxxxxx

Modifications

• ensure that all the children of /namespace/NAMESPACE are deleted, especially the ephemeral nodes for ownership
• ensure that on the code path that handles the deletion of the namespace we make progress, the main fact is about not relying on "cache.exists" before using a "delete": the new code is more pessimistic that goes with "deleteIfExists", handling "NotFound" gracefully
• add more logs about the delete namespace execution path

With this patch

• delete namespace seems to consistently work
• there are no more spammy logs about errors around the topic policies event reader (changelog_events...)
• in case we see an error we log at which point the procedure was (because we log all the paths that are deleted from zk)