KAZOO-2701 retry fax storage

store_attachment calls into attachment_url which will delete any attachments.  Due to the interaction between http_cache and bigcouch we often can not rely on the FS application response.  It frequently indicates failure when the attachment was successfully added.  Typically we verify the attachment to determine the status.

As such, it is very likely that the FS application reports the store failed (in the EXECUTE_COMPLETE event) when in reality it was successful.  This will then loop deleting it from the database and re-issuing a store_fax request, which if after 2 minutes will fail.  

Instead, I would propose you verify the attachment size on the database with that of the EXECUTE_COMPLETE event to determine if the store was successful.  I feel this will avoid accidentally deleting a valid attachment.