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

util/config/schema.py

@@ -15,6 +15,7 @@ INTERNAL_ONLY_PROPERTIES = {
   'SECSCAN_NOTIFICATION_QUEUE_NAME',
   'SECURITY_SCANNER_ISSUER_NAME',
   'NOTIFICATION_QUEUE_NAME',
+  'NAMESPACE_GC_QUEUE_NAME',
 
   'FEATURE_BILLING',
   'FEATURE_SUPPORT_CHAT',