Add a chunk cleanup queue for async GC of empty chunks

Instead of having the Swift storage engine try to delete the empty chunk(s) synchronously, we simply queue them and have a worker come along after 30s to delete the empty chunks. This has a few key benefits: it is async (doesn't slow down the push code), helps deal with Swift's eventual consistency (less retries necessary) and is generic for other storage engines if/when they need this as well

conf/init/service/chunkcleanupworker/log/run

@@ -0,0 +1,7 @@
+#!/bin/sh
+
+# Ensure dependencies start before the logger
+sv check syslog-ng > /dev/null || exit 1
+
+# Start the logger
+exec logger -i -t chunkcleanupworker

conf/init/service/chunkcleanupworker/run

@@ -0,0 +1,8 @@
+#! /bin/bash
+
+echo 'Starting chunk cleanup worker'
+
+cd /
+venv/bin/python -m workers.chunkcleanupworker 2>&1
+
+echo 'Chunk cleanup worker exited'