Expose each zone's last refresh time via rndc and the stats channel

When a zone is refreshed its file is touched by named. Checking the zone file's modification time is a good way to verify that secondary refreshes are working correctly. The last refresh time is good for this purpose since it can give the operator a reasonably prompt notification that somthing has broken with plenty of time to remediate, e.g. 24h after a zone stops updating is typically several days before the zone will break. If you try to alert based on the expiry time you either need to accommodate different expiry intervals in different zones (which is duplicating logic that already exists inside named), or set the alert to occur much closer to the time when a zone will break.

This patch adds the last refresh time alongside the other zone timing parameters in the output of rndc zonestatus, and it adds a similar selection of zone timing parameters to the statschannel output.

In servers that have a static configuration, named-checkconf -px is a handy way for a script to find out what zones are configured and where their files are, so that they can be checked for freshness. In a server with a dynamic configuration (using rndc addzone or catalog zones) the best way to get a list of zones is from the statschannel. This change gives monitoring scripts a simple one-stop shop for checking that all zones are reasonably fresh. For example, this script lists stale zones and when they were last refreshed:

curl -Ssf http://localhost:8053/json/v1/zones |
jq -r '.views[].zones[] |
	select(has("lastrefresh") and
		.lastrefresh < (now - 86400 | todate)) |
	"\(.lastrefresh) \(.name)"'