Don't send requests on a closed windmill Grpc streams

GrpcWindmillServer could send requests requests on client closed streams. This leads to windmill streams getting stalling occasionally for few seconds to few minutes. grpc-java doc says not to call onNext to send after a stream is client closed. 

When the streams get stalled it is logged as "Output channel stalled for {}s, outbound thread {}."  from here

Ref:

https://github.com/grpc/grpc-java/blob/master/stub/src/main/java/io/grpc/stub/StreamObserver.java#L62

https://github.com/apache/beam/blob/master/runners/google-cloud-dataflow-java/worker/src/main/java/org/apache/beam/runners/dataflow/worker/windmill/GrpcWindmillServer.java#L939