'Gets a single list of messages from all storage backends.'

def _get(self, *args, **kwargs):

    all_messages = []
    for storage in self.storages:
        (messages, all_retrieved) = storage._get()
        if (messages is None):
            break
        if messages:
            self._used_storages.add(storage)
        all_messages.extend(messages)
        if all_retrieved:
            break
    return (all_messages, all_retrieved)
