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.

server.go

@@ -1648,7 +1648,9 @@ func newServer(cfg *Config, listenAddrs []net.Addr,
 		chanNotifier: s.channelNotifier,
 		addrs:        s.addrSource,
 	}
-	backupFile := chanbackup.NewMultiFile(cfg.BackupFilePath)
+	backupFile := chanbackup.NewMultiFile(
+		cfg.BackupFilePath, cfg.NoBackupArchive,
+	)
 	startingChans, err := chanbackup.FetchStaticChanBackups(
 		s.chanStateDB, s.addrSource,
 	)