*:fix point-in-time backup

Backup process should be able to read all WALs until io.EOF to
generate a point-in-time backup.

Our WAL file is append-only. And the backup process will lock all
files before start reading, which can prevent the gc routine from
removing any files in the middle.

etcdctl/command/backup_command.go

@@ -66,7 +66,7 @@ func handleBackup(c *cli.Context) {
 		}
 	}
 
-	w, err := wal.OpenNotInUse(srcWAL, walsnap)
+	w, err := wal.OpenForRead(srcWAL, walsnap)
 	if err != nil {
 		log.Fatal(err)
 	}