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)

rpc/dataconn/heartbeatconn/heartbeatconn_test.go

@@ -0,0 +1,26 @@
+package heartbeatconn
+
+import (
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/zrepl/zrepl/rpc/dataconn/frameconn"
+)
+
+func TestFrameTypes(t *testing.T) {
+	assert.True(t, frameconn.IsPublicFrameType(heartbeat))
+}
+
+func TestNegativeTimer(t *testing.T) {
+
+	timer := time.NewTimer(-1 * time.Second)
+	defer timer.Stop()
+	time.Sleep(100 * time.Millisecond)
+	select {
+	case <-timer.C:
+		t.Log("timer with negative time fired, that's what we want")
+	default:
+		t.Fail()
+	}
+}