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)

Makefile

@@ -27,7 +27,7 @@ vendordeps:
 	dep ensure -v -vendor-only
 
 generate: #not part of the build, must do that manually
-	protoc -I=replication/pdu --go_out=replication/pdu replication/pdu/pdu.proto
+	protoc -I=replication/pdu --go_out=plugins=grpc:replication/pdu replication/pdu/pdu.proto
 	go generate -x ./...
 
 build:
@@ -38,7 +38,10 @@ test:
 	go test ./...
 
 vet:
-	go vet ./...
+	# for each supported platform to cover conditional compilation
+	GOOS=linux   go vet ./...
+	GOOS=darwin  go vet ./...
+	GOOS=freebsd go vet ./...
 
 $(ARTIFACTDIR):
 	mkdir -p "$@"