Renave `queue_adjust` to `on_idle` (#442)

httplib.h

@@ -367,9 +367,11 @@ class TaskQueue {
 public:
   TaskQueue() = default;
   virtual ~TaskQueue() = default;
+
   virtual void enqueue(std::function<void()> fn) = 0;
-  virtual void queue_adjust() { };
   virtual void shutdown() = 0;
+
+  virtual void on_idle(){};
 };
 
 class ThreadPool : public TaskQueue {
@@ -3498,7 +3500,7 @@ inline bool Server::listen_internal() {
       auto val = detail::select_read(svr_sock_, 0, 100000);
 
       if (val == 0) { // Timeout
-        task_queue->queue_adjust();
+        task_queue->on_idle();
         continue;
       }