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/workertask.cc

@@ -0,0 +1,48 @@
+/*
+ * Copyright (c) 2016 MariaDB Corporation Ab
+ *
+ * Use of this software is governed by the Business Source License included
+ * in the LICENSE.TXT file and at www.mariadb.com/bsl11.
+ *
+ * Change Date: 2019-07-01
+ *
+ * On the date above, in accordance with the Business Source License, use
+ * of this software will be governed by version 2 or later of the General
+ * Public License.
+ */
+
+#include "maxscale/workertask.hh"
+#include <maxscale/atomic.h>
+
+namespace maxscale
+{
+
+//
+// WorkerTask
+//
+WorkerTask::~WorkerTask()
+{
+}
+
+//
+// WorkerDisposableTask
+//
+WorkerDisposableTask::WorkerDisposableTask()
+    : m_count(0)
+{
+}
+
+void WorkerDisposableTask::inc_count()
+{
+    atomic_add(&m_count, 1);
+}
+
+void WorkerDisposableTask::dec_count()
+{
+    if (atomic_add(&m_count, -1) == 1)
+    {
+        delete this;
+    }
+}
+
+}