Add MaxFilesOrMaxDuration prune method

[Zylatis]

Currently we can prune old logs if either:

• there are >N of them, or
• some are older than a certain timestamp

However, there are situations where we want both to apply. For example we might want to say we want to keep 2 weeks of logs, unless this is going to fill up a hdd in which case prune them so we'd want to prune if either of these conditions are met.