google-cloud-datastore Add more_results value to query results

[mtso]

Please return the MoreResults query state value in query results. The end cursor to the next set of results will often return an empty result set, so this value could be used to determine whether or not the next page should be fetched and checked for pagination (and thus confidently return an empty next cursor if the next page is indeed empty as well). The QueryResultImpl only checks if the state value is not "NOT_FINISHED", not the other values as well.

See #3279