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/state.hh

@@ -175,6 +175,9 @@ struct Step
 
         /* The lowest ID of any build depending on this step. */
         BuildID lowestBuildID{std::numeric_limits<BuildID>::max()};
+
+        /* The time at which this step became runnable. */
+        system_time runnableSince;
     };
 
     std::atomic_bool finished{false}; // debugging
@@ -324,7 +327,8 @@ private:
     struct MachineType
     {
         unsigned int runnable{0}, running{0};
-        time_t lastActive{0};
+        system_time lastActive;
+        std::chrono::seconds waitTime; // time runnable steps have been waiting
     };
 
     Sync<std::map<std::string, MachineType>> machineTypes;
@@ -339,6 +343,8 @@ private:
         ~MachineReservation();
     };
 
+    std::atomic<time_t> lastDispatcherCheck{0};
+
 public:
     State();