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/CMakeLists.txt

@@ -44,6 +44,7 @@ add_library(maxscale-common SHARED
   users.cc
   utils.cc
   worker.cc
+  workertask.cc
   )
 
 if(WITH_JEMALLOC)