Enable drainer to purge old incremental backup data on disk

[sokada1221]

Feature Request

Is your feature request related to a problem? Please describe:

Currently, incremental backup data needs to be purged manually if drainer is configured to output files to disk.

Describe the feature you'd like:

Implement a retention period feature where drainer automatically purges data that is older than a configured time.
e.g. If 7 days is configured as a retention period, incremental backup data that is older than 7 days is automatically deleted to free up disk space.

Describe alternatives you've considered:

N/A

Teachability, Documentation, Adoption, Migration Strategy:

For example, a new configuration like the following can be added:

# Determines when to remove old data. Effective only if db-type is "file". Defaults to 7d.
retention-time = 30d

[suzaku]

Makes sense. If this feature is added with a new option retention-time, we'd better set the default value to something like -1 which means "Don't delete backup files for me", so that we don't surprise users of older versions.