Make namespace deletion asynchronous

Instead of deleting a namespace synchronously as before, we now mark the namespace for deletion, disable it, and rename it. A worker then comes along and deletes the namespace in the background. This results in a *significantly* better user experience, as the namespace deletion operation now "completes" in under a second, where before it could take 10s of minutes at the worse.

Fixes https://jira.coreos.com/browse/QUAY-838

endpoints/api/superuser_models_interface.py

@@ -353,7 +353,7 @@ class SuperuserDataInterface(object):
     """
 
   @abstractmethod
-  def delete_user(self, username):
+  def mark_user_for_deletion(self, username):
     """
     Returns None
     """
@@ -383,7 +383,7 @@ class SuperuserDataInterface(object):
     """
 
   @abstractmethod
-  def delete_organization(self, name):
+  def mark_organization_for_deletion(self, name):
     """
     Returns None
     """