clientv3: deprecate grpc.ErrClientConnClosing

[gyuho]

grpc.ErrClientConnClosing is being deprecated at grpc/grpc-go#1854.

https://github.com/coreos/etcd/blob/fe94f8f53af1c2ab684df713bf7e084ef3f1c675/clientv3/client.go#L550-L551

Just to not forget when we rewrite clientv3 balancer with new gRPC versions.

[dfawley]

Note that by the grpc-go versioning policy, we cannot and will not remove this*, as it would break backward compatibility.

However, it's possible small behavior changes will be necessary that cause us to return other errors instead of ErrClientConnClosing in various circumstances, so you should not typically rely on any specific errors unless the godoc is explicit about what will be returned.

• EDIT - until a MAJOR release (which may necessitate a new import path).

[gyuho]

@dfawley Thanks for update! We will keep that in mind.