channeldb+lnd: make channeldb backend configurable

This commit adds support for user configured channeldb backend.

config.go

@@ -1096,8 +1096,8 @@ func ValidateConfig(cfg Config, usageMessage string) (*Config, error) {
 
 // localDatabaseDir returns the default directory where the
 // local bolt db files are stored.
-func (c *config) localDatabaseDir() string {
-	return filepath.Join(cfg.DataDir,
+func (c *Config) localDatabaseDir() string {
+	return filepath.Join(c.DataDir,
 		defaultGraphSubDirname,
 		normalizeNetwork(activeNetParams.Name))
 }

lnd.go

@@ -250,11 +250,17 @@ func Main(cfg *Config, lisCfg ListenerCfg, shutdownChan <-chan struct{}) error {
 	ltndLog.Infof("Opening the main database, this might take a few " +
 		"minutes...")
 
+	chanDbBackend, err := cfg.DB.GetBackend(cfg.localDatabaseDir())
+	if err != nil {
+		ltndLog.Error(err)
+		return err
+	}
+
 	// Open the channeldb, which is dedicated to storing channel, and
 	// network related metadata.
 	startOpenTime := time.Now()
-	chanDB, err := channeldb.Open(
-		cfg.localDatabaseDir(),
+	chanDB, err := channeldb.CreateWithBackend(
+		chanDbBackend,
 		channeldb.OptionSetRejectCacheSize(cfg.Caches.RejectCacheSize),
 		channeldb.OptionSetChannelCacheSize(cfg.Caches.ChannelCacheSize),
 		channeldb.OptionSetSyncFreelist(cfg.SyncFreelist),

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,