Add detailed debug information when target_tables is empty

[ghost]

Summary

This pull request adds detailed debug information to the clean method of Cleaner to log the context when target_tables is empty. the following information will be logged when target_tables is empty:

• The target tables (tables)
• The except tables (@except_tables)
• All tables in the database (all_tables)

If you find this change to be excessive, it is perfectly fine not to merge it.

Other Information

Checklist

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[genya0407]

I welcome the addition of detailed logs, as they are helpful in the investigation. However, it seems a bit tedious to add logs each time they are needed.

For example, I thought the following gem could be introduced to mysql_rewinder and your project to make the log output more comprehensive.
https://github.com/pboling/debug_logging/tree/v3.1.9

I will discuss this a bit later on my end.