Deleting messages quits after one message

[navels]

This command:

slack-cleaner2 --token <my_token> --message --perform --as_user --channel <channel_name>

no longer deletes all the messages in the channel but instead just deletes the first message and quits.

I think this was broken in https://github.com/sgratzl/slack_cleaner2/commit/905274319268585eec6ef1180196629a939b63e4 probably because sleep_for is defaulting to None instead of 0.