added tp->wait_empty()

include/spdlog/details/thread_pool.h

@@ -139,6 +139,11 @@ public:
         return msg_counter_.load(std::memory_order_relaxed);
     }
 
+    void wait_empty()
+    {
+        q_.wait_empty();
+    }
+
 private:
     std::atomic<size_t> msg_counter_; // total # of messages processed in this pool
     q_type q_;