Fix issues in friendly-timestamp-string, add statusz

* Added statusz route
* `friendly-timestamp-string` was intermittently breaking on nil values (I couldn't determine why it wasn't happening consistently, it was very infrequent). I added a guard against nil values to prevent these.
* `friendly-timestamp-string` would display "0 days ago" when time ago was between 12 and 24 hours ago. I changed it to display "hours ago" until 24 hours.
Ah, good point. I'll add some documentation, unless you think the statusz page is unnecessary for watchman and should just be left out.