make jobs queue in the ThreadPool stable (i.e. FIFO for the same priority), otherwise some jobs can stay in queue untaken for a long time

src/Common/ThreadPool.h

@@ -131,7 +131,7 @@ private:
     bool threads_remove_themselves = true;
     const bool shutdown_on_exception = true;
 
-    boost::heap::priority_queue<JobWithPriority> jobs;
+    boost::heap::priority_queue<JobWithPriority,boost::heap::stable<true>> jobs;
     std::list<Thread> threads;
     std::exception_ptr first_exception;
     std::stack<OnDestroyCallback> on_destroy_callbacks;