rpc rewrite: control RPCs using gRPC + separate RPC for data transfer

transport/ssh: update go-netssh to new version
    => supports CloseWrite and Deadlines
    => build: require Go 1.11 (netssh requires it)

endpoint/context.go

@@ -9,6 +9,7 @@ type contextKey int
 
 const (
 	contextKeyLogger contextKey = iota
+	ClientIdentityKey
 )
 
 type Logger = logger.Logger