Introduce concept of Worker tasks

A Worker::Task is an object that can be sent to a worker for
execution. The task is sent to the worker using the messaging
mechanism where the `execute` function of the task will be
called in the thread context of the worker.

There are two kinds of tasks; regular tasks and disposable tasks.
The former are just sent to the worker for execution while the
latter are sent and subsequently disposed of, once the task has
been executed.

A disposable task can be sent to either one worker or to all
workers. In the latter case, the task will be deleted once it
has been executed by all workers.

A semaphore can be associated with a regular task. Once the task
has been executed by the worker, the semaphore will automatically
be posted. That way, it is trivial to send a task for execution
to a worker and wait until the task has been executed. For instance:

    Semaphore sem;
    MyTask task;

    pWorker->execute(&task, &sem);
    sem.wait();

    const MyResult& result = task.result();

The low level mechanism for posting and broadcasting messages will
be removed.

server/core/maxscale/worker.hh

@@ -13,14 +13,18 @@
  */
 
 #include <maxscale/cppdefs.hh>
+#include <memory>
 #include <maxscale/platform.h>
 #include "messagequeue.hh"
 #include "poll.h"
 #include "worker.h"
+#include "workertask.hh"
 
 namespace maxscale
 {
 
+class Semaphore;
+
 struct WORKER_STATISTICS
 {
     WORKER_STATISTICS()
@@ -61,6 +65,8 @@ class Worker : public MXS_WORKER
 
 public:
     typedef WORKER_STATISTICS STATISTICS;
+    typedef WorkerTask Task;
+    typedef WorkerDisposableTask DisposableTask;
 
     enum state_t
     {
@@ -244,6 +250,76 @@ public:
         return m_should_shutdown;
     }
 
+    /**
+     * Executes a task in the context of a Worker.
+     *
+     * @param pTask  The task to be executed.
+     * @param pSem   If non-NULL, will be posted once the task's `execute` return.
+     *
+     * @return True if the task could be *posted*, false otherwise.
+     *
+     * @attention  The instance must remain valid for as long as it takes for the
+     *             task to be transferred to the worker and its `execute` function
+     *             to be called.
+     *
+     * The semaphore can be used for waiting for the task to be finished.
+     *
+     * @code
+     *     Semaphore sem;
+     *     MyTask task;
+     *
+     *     pWorker->execute(&task, &sem);
+     *     sem.wait();
+     *
+     *     MyResult& result = task.result();
+     * @endcode
+     */
+    bool execute(Task* pTask, Semaphore* pSem = NULL);
+
+    /**
+     * Executes a disposable task in the context of a Worker.
+     *
+     * @param pTask  The task to be executed.
+     *
+     * @return True if the task could be *posted*, false otherwise.
+     *
+     * @attention  Once the task has been executed, it will be deleted.
+     */
+    bool execute(std::auto_ptr<DisposableTask> sTask);
+
+    /**
+     * Executes a task on all workers.
+     *
+     * @param pTask  The task to be executed.
+     * @param pSem   If non-NULL, will be posted once per worker when the task's
+     *               `execute` return.
+     *
+     * @return How many workers the task was posted to.
+     *
+     * @attention The very same task will be posted to all workers. The task
+     *            should either not have any sharable data or then it should
+     *            have data specific to each worker that can be accessed
+     *            without locks.
+     */
+    static size_t execute_on_all(Task* pTask, Semaphore* pSem = NULL);
+
+    /**
+     * Executes a task on all workers.
+     *
+     * @param pTask  The task to be executed.
+     *
+     * @return How many workers the task was posted to.
+     *
+     * @attention The very same task will be posted to all workers. The task
+     *            should either not have any sharable data or then it should
+     *            have data specific to each worker that can be accessed
+     *            without locks.
+     *
+     * @attention Once the task has been executed by all workers, it will
+     *            be deleted.
+     */
+    static size_t execute_on_all(std::auto_ptr<DisposableTask> sTask);
+
     /**
      * Post a message to a worker.
      *
@@ -337,6 +413,8 @@ private:
 
     static Worker* create(int id, int epoll_listener_fd);
 
+    bool execute_disposable(DisposableTask* pTask);
+
     void handle_message(MessageQueue& queue, const MessageQueue::Message& msg); // override
 
     static void thread_main(void* arg);