http: ping backend on healthcheck

Fixes #37.

http/routes.go

@@ -33,6 +33,10 @@ func handleError(err error) (int, error) {
 }
 
 func (s *Server) check(w http.ResponseWriter, r *http.Request, p httprouter.Params) (int, error) {
+	// Ping the backend if private tracker is enabled.
+	if err := s.tracker.Backend.Ping(); s.config.PrivateEnabled && err != nil {
+		return handleError(err)
+	}
 	_, err := w.Write([]byte("STILL-ALIVE"))
 	return handleError(err)
 }