Keep track of wait time per system type

I.e., how much time the currently runnable steps per system type have
been waiting. This is useful for deciding whether to provision more
machines.

src/hydra-queue-runner/hydra-queue-runner.cc

@@ -484,8 +484,11 @@ void State::dumpStatus(Connection & conn, bool log)
                 JSONObject nested2(out);
                 nested2.attr("runnable", i.second.runnable);
                 nested2.attr("running", i.second.running);
+                if (i.second.runnable > 0)
+                    nested2.attr("waitTime", i.second.waitTime.count() +
+                        i.second.runnable * (time(0) - lastDispatcherCheck));
                 if (i.second.running == 0)
-                    nested2.attr("lastActive", i.second.lastActive);
+                    nested2.attr("lastActive", std::chrono::system_clock::to_time_t(i.second.lastActive));
             }
         }
     }