Put service callback retries on a different queue

At the moment, if a service callback fails, it will get put on the retry queue.
This causes a potential problem though:

If a service's callback server goes down, we may generate a lot of retries and
this may then put a lot of items on the retry queue. The retry queue is also
responsible for other important parts of Notify such as retrying message
delivery and we don't want a service's callback server going down to have an
impact on the rest of Notify.

Putting the retries on a different queue means that tasks get processed
faster than if they were put back on the same 'service-callbacks' queue.

app/config.py

@@ -27,6 +27,7 @@ class QueueNames(object):
     PROCESS_FTP = 'process-ftp-tasks'
     CREATE_LETTERS_PDF = 'create-letters-pdf-tasks'
     CALLBACKS = 'service-callbacks'
+    CALLBACKS_RETRY = 'service-callbacks-retry'
     LETTERS = 'letter-tasks'
     SMS_CALLBACKS = 'sms-callbacks'
     ANTIVIRUS = 'antivirus-tasks'
@@ -50,6 +51,7 @@ class QueueNames(object):
             QueueNames.NOTIFY,
             QueueNames.CREATE_LETTERS_PDF,
             QueueNames.CALLBACKS,
+            QueueNames.CALLBACKS_RETRY,
             QueueNames.LETTERS,
             QueueNames.SMS_CALLBACKS,
             QueueNames.SAVE_API_EMAIL,