lncfg: add configuration for user specified db backend

This commit extends lncfg to support user specified database backend.
This supports configuration for both bolt and etcd (while only allowing
one or the other).

lnd.go

@@ -247,10 +247,6 @@ func Main(cfg *Config, lisCfg ListenerCfg, shutdownChan <-chan struct{}) error {
 	}
 
 	// Create the network-segmented directory for the channel database.
-	graphDir := filepath.Join(cfg.DataDir,
-		defaultGraphSubDirname,
-		normalizeNetwork(activeNetParams.Name))
-
 	ltndLog.Infof("Opening the main database, this might take a few " +
 		"minutes...")
 
@@ -258,7 +254,7 @@ func Main(cfg *Config, lisCfg ListenerCfg, shutdownChan <-chan struct{}) error {
 	// network related metadata.
 	startOpenTime := time.Now()
 	chanDB, err := channeldb.Open(
-		graphDir,
+		cfg.localDatabaseDir(),
 		channeldb.OptionSetRejectCacheSize(cfg.Caches.RejectCacheSize),
 		channeldb.OptionSetChannelCacheSize(cfg.Caches.ChannelCacheSize),
 		channeldb.OptionSetSyncFreelist(cfg.SyncFreelist),
@@ -493,7 +489,7 @@ func Main(cfg *Config, lisCfg ListenerCfg, shutdownChan <-chan struct{}) error {
 	var towerClientDB *wtdb.ClientDB
 	if cfg.WtClient.Active {
 		var err error
-		towerClientDB, err = wtdb.OpenClientDB(graphDir)
+		towerClientDB, err = wtdb.OpenClientDB(cfg.localDatabaseDir())
 		if err != nil {
 			err := fmt.Errorf("unable to open watchtower client "+
 				"database: %v", err)