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

config.py

@@ -137,6 +137,7 @@ class DefaultConfig(object):
   DOCKERFILE_BUILD_QUEUE_NAME = 'dockerfilebuild'
   REPLICATION_QUEUE_NAME = 'imagestoragereplication'
   SECSCAN_NOTIFICATION_QUEUE_NAME = 'security_notification'
+  CHUNK_CLEANUP_QUEUE_NAME = 'chunk_cleanup'
 
   # Super user config. Note: This MUST BE an empty list for the default config.
   SUPER_USERS = []