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/workertask.hh

@@ -0,0 +1,65 @@
+#pragma once
+/*
+ * 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/cppdefs.hh>
+
+namespace maxscale
+{
+
+class Worker;
+
+/**
+ * A WorkerTask represents a task to be performed by a Worker.
+ */
+class WorkerTask
+{
+public:
+    /**
+     * Destructor
+     */
+    virtual ~WorkerTask();
+
+    /**
+     * @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;
+};
+
+/**
+ * A WorkerDisposableTask represents a task to be performed by a Worker.
+ *
+ * When the task has been executed, the instance will automatically be
+ * deleted.
+ */
+class WorkerDisposableTask : public WorkerTask
+{
+protected:
+    WorkerDisposableTask();
+
+private:
+    friend class Worker;
+
+    void inc_count();
+    void dec_count();
+
+private:
+    int32_t m_count;
+};
+
+}