Fixing a number of scaling issues:

- Missed closing of file descriptor made ForkingTaskExecutor
  silently die after running out of FDs
- Tightened up scope for locks to prevent http timeout
- Simplified threadpool

daggyd/daggyd/daggyd.cpp

@@ -116,6 +116,7 @@ void daemonize()
 }
 
 namespace dl = daggy::loggers::dag_run;
+std::ofstream ofh;
 
 std::unique_ptr<dl::DAGRunLogger> loggerFactory(const rj::Value &config)
 {
@@ -138,7 +139,7 @@ std::unique_ptr<dl::DAGRunLogger> loggerFactory(const rj::Value &config)
       if (fn == "-")
         return std::make_unique<dl::OStreamLogger>(std::cout);
 
-      std::ofstream ofh(logConfig["file"].GetString());
+      ofh.open(logConfig["file"].GetString());
       return std::make_unique<dl::OStreamLogger>(ofh);
     }
   }