BigQuery: order not preserved when downloading ORDER BY query results to dataframe with BQ Storage API

[tswast]

Since data is downloaded in parallel, order might not be preserved. I believe if you explicitly request only a single stream, order is preserved. (Internal bug 130887748 to confirm this).

[tswast]

Idea: Add a _preserve_order property to RowIterator. If true, only use one stream. Defualt to false. Set to true in QueryJob.result() if the query string contains "ORDER BY" (case insensitive, any amount of whitespace between ORDER and BY).