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/local.py

@@ -14,7 +14,7 @@ logger = logging.getLogger(__name__)
 
 
 class LocalStorage(BaseStorageV2):
-  def __init__(self, metric_queue, storage_path):
+  def __init__(self, context, storage_path):
     super(LocalStorage, self).__init__()
     self._root_path = storage_path