Add ability to log "freshness_limit" and "current_age"

[c-taylor]

I want to log “freshness_limit” in ATS to allow us to better model the cache contents.
It would be good to enable the logging of “current_age” also.

This allows us to create logs for modeling cache age and churn over extended periods.

These would be new log fields:

• freshness_limit written when objects are served from or written to the cache.
• current_age when serving

[Ideally avoiding header use]