'gcloud.streaming': Consider overriding 'httplib2' download method

[tseaver]

See: #1210 (comment)

The comment in gcloud.streaming.transfer.Download.configure_request (inherited from apitools) says:

        # We send range requests because by default httplib2 stores entire
        # reponses in memory.  Consider overriding httplib2's download method
        # (as gsutil does) so that this is not necessary.

[theacodes]

Superseded by the proposal in #1346