A stream token that was previously sent by the server. The client should
set this field to the token from the most recent WriteResponse it has
received. This acknowledges that the client has received responses up to
this token. After sending this token, earlier tokens may not be used
anymore. The server may close the stream if there are too many
unacknowledged responses. Leave this field unset when creating a new
stream. To resume a stream at a specific point, set this field and the
stream_id field. Leave this field unset when creating a new stream.