Accept auto_ptr<T> where T is derived type

Without the member template it is not possible to pass an auto_ptr
instantiated with a derived type to post() or broadcast().

The reason is that the conversion constructor and conversion
operator of auto_ptr are equally good for that purpose, and hence
the compilation ends with an error.

server/core/maxscale/worker.hh

@@ -298,6 +298,12 @@ public:
      */
     bool post(std::auto_ptr<DisposableTask> sTask, enum execute_mode_t mode = EXECUTE_AUTO);
 
+    template<class T>
+    bool post(std::auto_ptr<T> sTask, enum execute_mode_t mode = EXECUTE_AUTO)
+    {
+        return post(std::auto_ptr<DisposableTask>(sTask.release()), mode);
+    }
+
     /**
      * Posts a task to all workers for execution.
      *
@@ -331,6 +337,12 @@ public:
      */
     static size_t broadcast(std::auto_ptr<DisposableTask> sTask);
 
+    template<class T>
+    static size_t broadcast(std::auto_ptr<T> sTask)
+    {
+        return broadcast(std::auto_ptr<DisposableTask>(sTask.release()));
+    }
+
     /**
      * Executes a task on all workers in serial mode (the task is executed
      * on at most one worker thread at a time). When the function returns

server/core/maxscale/workertask.hh

@@ -49,17 +49,18 @@ public:
  */
 class WorkerDisposableTask
 {
+public:
+    /**
+     * Destructor
+     */
+    virtual ~WorkerDisposableTask();
+
 protected:
     /**
      * Constructor
      */
     WorkerDisposableTask();
 
-    /**
-     * Destructor
-     */
-    virtual ~WorkerDisposableTask();
-
     /**
      * @brief Called in the context of a specific worker.
      *