BigQuery: fetch_data() should return a core iterator

[tswast]

See: https://github.com/GoogleCloudPlatform/google-cloud-python/blob/master/bigquery/google/cloud/bigquery/query.py#L363

def fetch_data(...):
    """...
     :returns: ``(row_data, total_rows, page_token)``
    ...
    """

I would expect it to return some kind of Iterator to be consistent with the other cloud APIs.

[dhermes]

I'll defer to @tseaver. We discussed this for QueryResults.fetch_data and one other method?

[tseaver]

Table.fetch_data already returns an iterator. I will work on a PR to make QueryResults.fetch_data do likewise (I thought it was already done).