Advanced logging: ignore changes to civicrm_group refresh_date field

I've come across a Civi with 94 million records in the log_civicrm_group table. Investigating why, I found that although changes to cache_date were being excluded from logging, changes to refresh_date, which are similarly frequent, were included. This led me to:
CRM-13028 Adv logging: Refine triggers for tables with datestamps
where a change was made to exclude civicrm_group cache_date from logging. I think it makes sense also to exclude refresh_date.

I've discussed with Brian Shaughnessy, who worked on CRM-13028. He agrees that refresh_date should be excluded and suggested opening a new issue rather than reopen that one.

I'll do a pull request. I imagine this will be wanted against 4.6 but it would be good to get this into 4.4 too.