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

libdaggy/tests/unit_threadpool.cpp

@@ -15,14 +15,12 @@ TEST_CASE("threadpool", "[threadpool]")
 
   SECTION("Adding large tasks queues with return values")
   {
-    auto tq = std::make_shared<daggy::TaskQueue>();
     std::vector<std::future<uint32_t>> res;
     for (size_t i = 0; i < 100; ++i)
-      res.emplace_back(tq->addTask([&cnt]() {
+      res.emplace_back(tp.addTask([&cnt]() {
         cnt++;
         return cnt.load();
       }));
-    tp.addTasks(tq);
     for (auto &r : res)
       r.get();
     REQUIRE(cnt == 100);