[Swift] Fix 503 error when get queue stats

when use swift backend, if there is only one message in a queue,
then try to get the queue 's stats, Zaqar will raise 503 error.

It's because that Swift backend used shallow copy which is wrong.

Closes-bug: #1686021

Change-Id: Ie563ff63ea90e7011a6ece525f5e7795109ba1d0

zaqar/storage/swift/messages.py

@@ -11,6 +11,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import copy
 import functools
 import uuid
 
@@ -329,7 +330,7 @@ class MessageQueueHandler(object):
                     'age': now - created,
                     'created': timeutils.iso8601_from_timestamp(created)}
                 if oldest is None:
-                    oldest = newest
+                    oldest = copy.deepcopy(newest)
                 total += 1
                 if headers.get('x-object-meta-claimid'):
                     claimed += 1