multi: add ChannelGraph Config struct

And use this struct to pass NewChannelGraph anything it needs to be able
to init the KVStore that it houses. This will allow us to add
ChannelGraph specific options.

graph/db/graph_test.go

@@ -4005,7 +4005,7 @@ func TestGraphLoading(t *testing.T) {
 	defer backend.Close()
 	defer backendCleanup()
 
-	graph, err := NewChannelGraph(backend)
+	graph, err := NewChannelGraph(&Config{KVDB: backend})
 	require.NoError(t, err)
 
 	// Populate the graph with test data.
@@ -4015,7 +4015,7 @@ func TestGraphLoading(t *testing.T) {
 
 	// Recreate the graph. This should cause the graph cache to be
 	// populated.
-	graphReloaded, err := NewChannelGraph(backend)
+	graphReloaded, err := NewChannelGraph(&Config{KVDB: backend})
 	require.NoError(t, err)
 
 	// Assert that the cache content is identical.