Reset nodes in frontend when scope-app restarted

When the scope-app restarts, it no longer has a
reference to the previous node set. Therefore,
the delta update adds *all* nodes but does not
remove legacy ones.

`reset==true` tells the frontend to start fresh.

Fixes #2708

render/detailed/topology_diff.go

@@ -10,11 +10,12 @@ type Diff struct {
 	Add    []NodeSummary `json:"add"`
 	Update []NodeSummary `json:"update"`
 	Remove []string      `json:"remove"`
+	Reset  bool          `json:"reset,omitempty"`
 }
 
 // TopoDiff gives you the diff to get from A to B.
 func TopoDiff(a, b NodeSummaries) Diff {
-	diff := Diff{}
+	diff := Diff{Reset: a == nil}
 
 	notSeen := map[string]struct{}{}
 	for k := range a {

render/detailed/topology_diff_test.go

@@ -79,6 +79,22 @@ func TestTopoDiff(t *testing.T) {
 				Update: []detailed.NodeSummary{nodeap},
 			},
 		},
+		{
+			label: "no previous nodes",
+			have:  detailed.TopoDiff(nil, nodes(nodea)),
+			want: detailed.Diff{
+				Add:   []detailed.NodeSummary{nodea},
+				Reset: true,
+			},
+		},
+		{
+			label: "empty previous nodes",
+			have:  detailed.TopoDiff(nodes(), nodes(nodea)),
+			want: detailed.Diff{
+				Add:   []detailed.NodeSummary{nodea},
+				Reset: false,
+			},
+		},
 	} {
 		sort.Strings(c.have.Remove)
 		sort.Sort(ByID(c.have.Add))