graph/db: init KVStore outside of ChannelGraph

So that we can pass in the abstract V1Store in preparation for adding a
SQL implementation of the KVStore.

autopilot/prefattach_test.go

@@ -47,7 +47,10 @@ func newDiskChanGraph(t *testing.T) (testGraph, error) {
 	})
 	require.NoError(t, err)
 
-	graphDB, err := graphdb.NewChannelGraph(&graphdb.Config{KVDB: backend})
+	graphStore, err := graphdb.NewKVStore(backend)
+	require.NoError(t, err)
+
+	graphDB, err := graphdb.NewChannelGraph(graphStore)
 	require.NoError(t, err)
 
 	require.NoError(t, graphDB.Start())