multi: let chan and graph db implement AddrSource

Then use both to construct a multiAddrSource AddrSource and use that
around the code-base.

graph/db/graph.go

@@ -414,6 +414,32 @@ func (c *ChannelGraph) NewPathFindTx() (kvdb.RTx, error) {
 	return c.db.BeginReadTx()
 }
 
+// AddrsForNode returns all known addresses for the target node public key that
+// the graph DB is aware of. The returned boolean indicates if the given node is
+// unknown to the graph DB or not.
+//
+// NOTE: this is part of the channeldb.AddrSource interface.
+func (c *ChannelGraph) AddrsForNode(nodePub *btcec.PublicKey) (bool, []net.Addr,
+	error) {
+
+	pubKey, err := route.NewVertexFromBytes(nodePub.SerializeCompressed())
+	if err != nil {
+		return false, nil, err
+	}
+
+	node, err := c.FetchLightningNode(pubKey)
+	// We don't consider it an error if the graph is unaware of the node.
+	switch {
+	case err != nil && !errors.Is(err, ErrGraphNodeNotFound):
+		return false, nil, err
+
+	case errors.Is(err, ErrGraphNodeNotFound):
+		return false, nil, nil
+	}
+
+	return true, node.Addresses, nil
+}
+
 // ForEachChannel iterates through all the channel edges stored within the
 // graph and invokes the passed callback for each edge. The callback takes two
 // edges as since this is a directed graph, both the in/out edges are visited.