reminder: config cleanup opportunities for 3.x series

[majewsky]

The following changes would make sense to do when we're breaking compatibility anyway, but don't justify breaking compatibility by themselves:

• move the filename-based filters (only, except, immutable) into jobs[].match (possibly into a subsection like jobs[].match.path)
• rename jobs[].expiration.delay_seconds to just jobs[].expiration.delay and support the nicer value syntax from jobs[].match.not_older_than