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

conf/init/service/batch/namespacegcworker/run

@@ -0,0 +1,9 @@
+#! /bin/bash
+
+echo 'Starting Namespace GC worker'
+
+QUAYPATH=${QUAYPATH:-"."}
+cd ${QUAYDIR:-"/"}
+PYTHONPATH=$QUAYPATH venv/bin/python -m workers.namespacegcworker 2>&1
+
+echo 'Namespace GC exited'