box: allow to specify the checkpoint to backup

Even though we store last box.cfg.checkpoint_count checkpoints, we can
only restore to the last one, because the backup API only allows to
backup the last checkpoint. This patch adds an optional argument to
box.backup.start() which specifies the index of the checkpoint to
backup. If it is omitted or is 0, box.backup.start() will return files
corresponding to the last checkpoint. If it is 1, it will back the
previous checkpoint, and so on.

Closes #3410