Avoid premature wakeup from reader thread by doing the notify AFTER the mutex is released

http://www.justsoftwaresolutions.co.uk/threading/implementing-a-thread-safe-queue-using-condition-variables.html

g2log/src/shared_queue.hpp

@@ -35,8 +35,10 @@ public:
   shared_queue(){}
 
   void push(T item){
-    std::lock_guard<std::mutex> lock(m_);
-    queue_.push(std::move(item));
+    {  
+      std::lock_guard<std::mutex> lock(m_);
+      queue_.push(std::move(item));
+    }
     data_cond_.notify_one();
   }