Add concurrent execution helper to Worker

Concurrently executing a task on all workers *and* waiting until
all workers have executed the task seems to be common enough to
warrant a helper function for that purpose.

server/core/maxscale/worker.hh

@@ -338,6 +338,16 @@ public:
      */
     static size_t execute_on_all_serially(Task* pTask);
 
+    /**
+     * Executes a task on all workers concurrently and waits until
+     * all workers are done.
+     *
+     * @param pTask  The task to be executed.
+     *
+     * @return How many workers the task was posted to.
+     */
+    static size_t execute_on_all_concurrently(Task* pTask);
+
     /**
      * Post a message to a worker.
      *