box: require box.cfg.checkpoint_count to be >= 1

We must store at least one snapshot, otherwise we wouldn't recover
after restart, so if checkpoint_count is set to 0, we disable garbage
collection. This contravenes the notion followed everywhere else in
tarantool: if we want an option value (timeout, checkpoint count, etc)
to be infinite, we should set it to a very big number, not to 0.
Make checkpoint_count comply.