Revert "Re-enable bundling (for consideration)"

[tseaver]

Reverts #1950

Apparently, we have a much tinier fix possible.

[daspecster]

Ah, this still has to be confirmed right?

[tseaver]

Yup. But I wanted to get ready, as my memory of something @geigerj said a week ago makes me think we're going to back it out.

[tseaver]

@geigerj, @bjwatson

        options = CallOptions(is_bundling=True)
        message_pbs = [_message_pb_from_dict(message)
                       for message in messages]
        try:
            result = self._gax_api.publish(topic_path, message_pbs,
                                           options=options)
        except GaxError as exc:
            if exc_to_code(exc.cause) == StatusCode.NOT_FOUND:
                raise NotFound(topic_path)
            raise
        return result.message_ids

fails because the GAX publish wrapper returns an Event object when is_bundling is set True:

======================================================================
ERROR: test_message_pull_mode_e2e (pubsub.TestPubsub)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/tseaver/projects/agendaless/Google/src/gcloud-python/system_tests/pubsub.py", line 157, in test_message_pull_mode_e2e
    topic.publish(MESSAGE_1, extra=EXTRA_1)
  File "/home/tseaver/projects/agendaless/Google/src/gcloud-python/gcloud/pubsub/topic.py", line 246, in publish
    message_ids = api.topic_publish(self.full_name, [message_data])
  File "/home/tseaver/projects/agendaless/Google/src/gcloud-python/gcloud/pubsub/_gax.py", line 175, in topic_publish
    return result.message_ids
AttributeError: 'Event' object has no attribute 'message_ids'

[tseaver]

@bjwatson I believe we actually want _gax._PublisherAPI.topic_publish to disable bundling: it needs to remain semantically equivalent to the JSON-over-API version (i.e, returning the message_ids), which makes the async bundling bit less useful.

[tseaver]

@daspecster I think we should go ahead and merge the revert.

[daspecster]

@tseaver ok, LGTM!