CallOptions should accept 'page_token' field as a parameter to start a stream from a specific position
PagedEnumerable should provide the interface to access to the current response object and the next page token in the current response (they are accessible through page instance, but a bit obscure).
remove is_page_streaming from CallOptions. page-streaming methods should always return PagedEnumerable.
From https://github.com/googleapis/toolkit/issues/139:
page
instance, but a bit obscure).