Do not inherit WorkerDisposableTask from WorkerTask

WorkerDisposableTask is intended to be passed using auto_ptr, to
make it plain that the ownership is moved. If it's inherited from
WorkerTask it can be passed as a WorkerTask, which is confusing as
it in that case will not be disposed of.

server/core/maxscale/workertask.hh

@@ -47,11 +47,29 @@ public:
  * When the task has been executed, the instance will automatically be
  * deleted.
  */
-class WorkerDisposableTask : public WorkerTask
+class WorkerDisposableTask
 {
 protected:
+    /**
+     * Constructor
+     */
     WorkerDisposableTask();
 
+    /**
+     * Destructor
+     */
+    virtual ~WorkerDisposableTask();
+
+    /**
+     * @brief Called in the context of a specific worker.
+     *
+     * @param worker  The worker in whose context `execute` is called.
+     *
+     * @attention As the function is called by a worker, the body of `execute`
+     *            should execute quickly and not perform any blocking operations.
+     */
+    virtual void execute(Worker& worker) = 0;
+
 private:
     friend class Worker;
 

server/core/workertask.cc

@@ -33,6 +33,10 @@ WorkerDisposableTask::WorkerDisposableTask()
 {
 }
 
+WorkerDisposableTask::~WorkerDisposableTask()
+{
+}
+
 void WorkerDisposableTask::inc_ref()
 {
     atomic_add(&m_count, 1);