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

storage/fakestorage.py

@@ -9,7 +9,7 @@ from storage.basestorage import BaseStorageV2
 _FAKE_STORAGE_MAP = defaultdict(StringIO.StringIO)
 
 class FakeStorage(BaseStorageV2):
-  def __init__(self, metric_queue):
+  def __init__(self, context):
     super(FakeStorage, self).__init__()
 
   def _init_path(self, path=None, create=False):