channeldb+lnd: make channeldb backend configurable

This commit adds support for user configured channeldb backend.

server.go

@@ -377,8 +377,7 @@ func newServer(cfg *Config, listenAddrs []net.Addr, chanDB *channeldb.DB,
 
 	// Initialize the sphinx router, placing it's persistent replay log in
 	// the same directory as the channel graph database.
-	graphDir := chanDB.Path()
-	sharedSecretPath := filepath.Join(graphDir, "sphinxreplay.db")
+	sharedSecretPath := filepath.Join(cfg.localDatabaseDir(), "sphinxreplay.db")
 	replayLog := htlcswitch.NewDecayedLog(sharedSecretPath, cc.chainNotifier)
 	sphinxRouter := sphinx.NewRouter(
 		nodeKeyECDH, activeNetParams.Params, replayLog,