graph/db: let ChannelGraph init the graphCache

In this commit, we let the ChannelGraph be responsible for populating
the graphCache and then passing it to the KVStore. This is a first step
in moving the graphCache completely out of the KVStore layer.

graph/notifications_test.go

@@ -1093,12 +1093,10 @@ func makeTestGraph(t *testing.T, useCache bool) (*graphdb.ChannelGraph,
 
 	t.Cleanup(backendCleanup)
 
-	graph, err := graphdb.NewChannelGraph(&graphdb.Config{
-		KVDB: backend,
-		KVStoreOpts: []graphdb.KVStoreOptionModifier{
-			graphdb.WithUseGraphCache(useCache),
-		},
-	})
+	graph, err := graphdb.NewChannelGraph(
+		&graphdb.Config{KVDB: backend},
+		graphdb.WithUseGraphCache(useCache),
+	)
 	if err != nil {
 		return nil, nil, err
 	}