Keep stats for the Hydra auto scaler

"hydra-queue-runner --status" now prints how many runnable and running
build steps exist for each machine type. This allows additional
machines to be provisioned based on the Hydra load.

src/hydra-queue-runner/counter.hh

@@ -8,5 +8,5 @@ struct MaintainCount
 {
     counter & c;
     MaintainCount(counter & c) : c(c) { c++; }
-    ~MaintainCount() { c--; }
+    ~MaintainCount() { auto prev = c--; assert(prev); }
 };