Move execute_worker_task to MonitorInstance

The function is rather general and may of use to other monitor modules.

server/core/monitor.cc

@@ -3013,4 +3013,45 @@ void MonitorInstance::run_one_tick()
     store_server_journal(m_monitor, m_master);
 }
 
+bool MonitorInstance::execute_worker_task(GenericFunction func, execute_mode_t mode)
+{
+    /* The worker message system works on objects of class Task, each representing a different action.
+     * Let's use a function object inside a task to construct a generic action. */
+    class CustomTask : public maxscale::Worker::Task
+    {
+    public:
+        CustomTask(GenericFunction func)
+            : m_func(func)
+        {}
+
+    private:
+        GenericFunction m_func;
+        void execute(maxscale::Worker& worker)
+        {
+            m_func();
+            delete this; // Ok, since this object is not touched afterwards.
+        }
+    };
+
+    CustomTask* task = new (std::nothrow) CustomTask(func);
+    bool sent = false;
+
+    if (mode == Worker::EXECUTE_AUTO)
+    {
+        maxscale::Semaphore done(0);
+        /* Although the current method is being ran in the admin thread, 'post' sends the task to the
+         * worker thread of "this". */
+        sent = post(task, &done, mode);
+        if (sent)
+        {
+            done.wait();
+        }
+    }
+    else
+    {
+        sent = post(task, NULL, mode);
+    }
+    return sent;
+}
+
 }