TimerServer: use task_dispatcher.add_task instead of high level add_task

create_server returns either a BaseWSGIServer or a MultiSocketServer,
but add_task only exists in BaseWSGIServer - and the implementation is
simply [1]:

    def add_task(self, task):
        self.task_dispatcher.add_task(task)

by using self.task_dispatcher.add_task directly, we are compatible with
both cases.

This is relevant when running timerserver with runUnitTest, where we
create multiple sockets, one for http and one for web-dav.

1: https://github.com/Pylons/waitress/blob/v1.4.4/src/waitress/server.py#L290-L291

product/TimerService/timerserver/TimerServer.py

@@ -65,7 +65,7 @@ class TimerServer(threading.Thread):
                         _module_name=module_name,
                         _request=request,
                         _response=response)
-                    server.add_task(self)
+                    server.task_dispatcher.add_task(self)
 
                 def cancel(self):
                     pass