lnd+chanbackup: add lnd config flag

In this commit, we add the --no-backup-archive with a default
as false. When set to true then previous channel backup file will
not be archived but replaced. We also modify TestUpdateAndSwap
test to make sure the new behaviour works as expected.

chanbackup/backupfile.go

@@ -54,12 +54,15 @@ type MultiFile struct {
 
 	// archiveDir is the directory where we'll store old channel backups.
 	archiveDir string
+
+	// noBackupArchive indicates whether old backups should be deleted
+	// rather than archived.
+	noBackupArchive bool
 }
 
 // NewMultiFile create a new multi-file instance at the target location on the
 // file system.
-func NewMultiFile(fileName string) *MultiFile {
-
+func NewMultiFile(fileName string, noBackupArchive bool) *MultiFile {
 	// We'll our temporary backup file in the very same directory as the
 	// main backup file.
 	backupFileDir := filepath.Dir(fileName)
@@ -71,15 +74,17 @@ func NewMultiFile(fileName string) *MultiFile {
 	)
 
 	return &MultiFile{
-		fileName:     fileName,
-		tempFileName: tempFileName,
-		archiveDir:   archiveDir,
+		fileName:        fileName,
+		tempFileName:    tempFileName,
+		archiveDir:      archiveDir,
+		noBackupArchive: noBackupArchive,
 	}
 }
 
 // UpdateAndSwap will attempt write a new temporary backup file to disk with
 // the newBackup encoded, then atomically swap (via rename) the old file for
-// the new file by updating the name of the new file to the old.
+// the new file by updating the name of the new file to the old. It also checks
+// if the old file should be archived first before swapping it.
 func (b *MultiFile) UpdateAndSwap(newBackup PackedMulti) error {
 	// If the main backup file isn't set, then we can't proceed.
 	if b.fileName == "" {
@@ -172,7 +177,13 @@ func (b *MultiFile) ExtractMulti(keyChain keychain.KeyRing) (*Multi, error) {
 // specified archive directory, and copies the contents of the main backup file
 // to the new archive file.
 func (b *MultiFile) createArchiveFile() error {
-	// We check for old channel backup file first.
+	// User can skip archiving of old backup files to save disk space.
+	if b.noBackupArchive {
+		log.Debug("Skipping archive of old backup file as configured")
+		return nil
+	}
+
+	// Check for old channel backup file.
 	oldFileExists := lnrpc.FileExists(b.fileName)
 	if !oldFileExists {
 		log.Debug("No old channel backup file to archive")