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.
Where does this file come from? If someone else uses watchman, they won't know what this is, so we should at least provide some documentation as to what it is.