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)

util/bytecounter/bytecounter_streamcopier_test.go

@@ -0,0 +1,38 @@
+package bytecounter
+
+import (
+	"io"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/zrepl/zrepl/zfs"
+)
+
+type mockStreamCopierAndReader struct {
+	zfs.StreamCopier // to satisfy interface
+	reads            int
+}
+
+func (r *mockStreamCopierAndReader) Read(p []byte) (int, error) {
+	r.reads++
+	return len(p), nil
+}
+
+var _ io.Reader = &mockStreamCopierAndReader{}
+
+func TestNewStreamCopierReexportsReader(t *testing.T) {
+	mock := &mockStreamCopierAndReader{}
+	x := NewStreamCopier(mock)
+
+	r, ok := x.(io.Reader)
+	if !ok {
+		t.Fatalf("%T does not implement io.Reader, hence reader cannout have been wrapped", x)
+	}
+
+	var buf [23]byte
+	n, err := r.Read(buf[:])
+	assert.True(t, mock.reads == 1)
+	assert.True(t, n == len(buf))
+	assert.NoError(t, err)
+	assert.True(t, x.Count() == 23)
+}