Allow serial execution of worker tasks

The Worker::execute_on_all_wait is intended to be used with dcb_foreach
which expects a single-threaded context for its function.

server/core/maxscale/worker.hh

@@ -320,6 +320,21 @@ public:
      */
     static size_t execute_on_all(std::auto_ptr<DisposableTask> sTask);
 
+    /**
+     * Executes a task on all workers in serial mode.
+     *
+     * The task is executed on at most one worker thread at a time.
+     *
+     * @param pTask  The task to be executed.
+     *
+     * @return How many workers the task was posted to.
+     *
+     * @warning This function is extremely inefficient and will be slow compared
+     * to the other functions. Only use this function when printing thread-specific
+     * data to stdout.
+     */
+    static size_t execute_on_all_serially(Task* pTask);
+
     /**
      * Post a message to a worker.
      *