config: expose the InvoiceDB interface in DatabaseInstances

By now the only implementation is still the one in channeldb but other
project can use their custom implementations if needed.

server.go

@@ -614,7 +614,7 @@ func newServer(cfg *Config, listenAddrs []net.Addr,
 		uint32(currentHeight), currentHash, cc.ChainNotifier,
 	)
 	s.invoices = invoices.NewRegistry(
-		dbs.ChanStateDB, expiryWatcher, &registryConfig,
+		dbs.InvoiceDB, expiryWatcher, &registryConfig,
 	)
 
 	s.htlcNotifier = htlcswitch.NewHtlcNotifier(time.Now)