Worker executor adjustments

- Reset the notify_timeout on start (to ensure that the
  notification thread will continue working on repeated
  stops/starts)
- Adjust comment of stop/start method to reflect that more
  than just the proxy thread is started and stopped.

Fixes bug 1300373

Change-Id: I534fa87d9f3b9cfd7aa553f157cb7e776a08f54f

taskflow/engines/worker_based/executor.py

@@ -190,16 +190,19 @@ class WorkerTaskExecutor(executor.TaskExecutorBase):
         return async_utils.wait_for_any(fs, timeout)
 
     def start(self):
-        """Start proxy thread."""
+        """Start proxy thread (and associated topic notification thread)."""
         if self._proxy_thread is None:
             self._proxy_thread = self._make_thread(self._proxy.start)
             self._proxy_thread.start()
             self._proxy.wait()
+            self._notify_timeout.reset()
             self._notify_thread = self._make_thread(self._notify_topics)
             self._notify_thread.start()
 
     def stop(self):
-        """Stop proxy, so its thread would be gracefully terminated."""
+        """Stop proxy thread (and associated topic notification thread), so
+        those threads will be gracefully terminated.
+        """
         if self._proxy_thread is not None:
             if self._proxy_thread.is_alive():
                 self._notify_timeout.interrupt()