Improve RoutingWorker utilities

Added an overload to execute_concurrently that takes an std::function as a
parameter and added a const version of operator* for rworker_local. Also
removed the std::move of the return value in rworker_local::values as it
can prevent RVO from taking place.

server/core/routingworker.cc

@@ -780,6 +780,13 @@ size_t RoutingWorker::execute_concurrently(Task& task)
     return sem.wait_n(RoutingWorker::broadcast(&task, &sem));
 }
 
+// static
+size_t RoutingWorker::execute_concurrently(std::function<void()> func)
+{
+    Semaphore sem;
+    return sem.wait_n(RoutingWorker::broadcast(func, &sem, EXECUTE_AUTO));
+}
+
 // static
 size_t RoutingWorker::broadcast_message(uint32_t msg_id, intptr_t arg1, intptr_t arg2)
 {