ServerRequestStreamObserver is not properly closed

ServerRequestStreamObserver is not properly closed.

After receiving the COMPLETE flag from the client side, it immediately closes the stream and also sends a COMPLETE flag to the client. see [1]. However, the server observer may not finish processing requests in onNext, therefore, the replies of these unfinished requests are lost.

If the client side waits on receiving all replies[2], this situation will become a deadlock.

[1] https://github.com/apache/ratis/blob/master/ratis-grpc/src/main/java/org/apache/ratis/grpc/server/GrpcServerProtocolService.java#L143-L147
[2] https://github.com/apache/ratis/blob/master/ratis-grpc/src/main/java/org/apache/ratis/grpc/server/GrpcLogAppender.java#L624