[Issue 1232] Export zone timers via stats channel
This patch adds the loaded, expires, and retry timers to the zone info in the statistics channel. This makes it much easier to monitor for zones that are due to expire soon. The code seems to build cleanly, and the server passes
make check. I'm open to adjusting the formatting of these or making additional code modifications.
The same basic code works in 9.11 and 9.14 as well, so this could be easily backported.