status code for unclean connection termination

When the TCP connection underlying a netty server connection is abruptly closed by the client, grpc-java goes through all active call streams and closes them as `UNAVAILABLE`:
https://github.com/grpc/grpc-java/blob/6518d7bd6dc496c89ff320e06c73424f26af364a/netty/src/main/java/io/grpc/netty/NettyServerHandler.java#L613-L625 A problem we've seen with this arrangement is that there is no way for to distinguish `UNAVAILABLE` call failures due to "real" server unavailability and due to the client uncleanly closing their connections (probably due to abrupt process death). What do you think about changing the status in this situation to `CANCELLED`?

NettyServerHandler.java の613～625行目から始め、クライアントの突然の終了をどのように表現すべきかについての14件のコメントによる議論を読んでください。CANCELLED が意味的に正しいかどうかについて合意し、それに対応する動作変更と検証計画を定義できれば完了です。

索引モデルが issue の本文から書いたものです。