graph/db: split HasNode into two methods

HasNode currently is very v1 specific since it returns a time.Time
timestamp which is specific to V1 node announcements. However, it is
mostly only used for the "exists" return value. So here we split it up
into HasNode which just checks existence and HasV1Node which retains the
same behavaiour as before.

graph/db/graph_test.go

@@ -138,7 +138,7 @@ func TestNodeInsertionAndDeletion(t *testing.T) {
 	dbNode, err := graph.FetchNode(ctx, testPub)
 	require.NoError(t, err, "unable to locate node")
 
-	_, exists, err := graph.HasNode(ctx, dbNode.PubKeyBytes)
+	exists, err := graph.HasNode(ctx, dbNode.PubKeyBytes)
 	require.NoError(t, err)
 	require.True(t, exists)
 
@@ -309,7 +309,7 @@ func TestPartialNode(t *testing.T) {
 	dbNode2, err := graph.FetchNode(ctx, pubKey2)
 	require.NoError(t, err)
 
-	_, exists, err := graph.HasNode(ctx, dbNode1.PubKeyBytes)
+	exists, err := graph.HasNode(ctx, dbNode1.PubKeyBytes)
 	require.NoError(t, err)
 	require.True(t, exists)
 
@@ -318,7 +318,7 @@ func TestPartialNode(t *testing.T) {
 	expectedNode1 := models.NewV1ShellNode(pubKey1)
 	compareNodes(t, expectedNode1, dbNode1)
 
-	_, exists, err = graph.HasNode(ctx, dbNode2.PubKeyBytes)
+	exists, err = graph.HasNode(ctx, dbNode2.PubKeyBytes)
 	require.NoError(t, err)
 	require.True(t, exists)