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
Leave this field unset when creating a new stream.