Always shutdown worker thread pool (#965)

With this commit we ensure that the worker's thread pool is always
shutdown when its correspdonding actor exits. We also emit an additional
log message after the pool has been shutdown to clearly indicate that
the actor is exiting now.

esrally/driver/driver.py

@@ -844,10 +844,11 @@ def receiveMsg_WakeupMessage(self, msg, sender):
                 self.wakeupAfter(datetime.timedelta(seconds=self.wakeup_interval))
 
     def receiveMsg_ActorExitRequest(self, msg, sender):
-        self.logger.info("Worker[%s] is exiting due to ActorExitRequest.", str(self.worker_id))
+        self.logger.info("Worker[%s] has received ActorExitRequest.", str(self.worker_id))
         if self.executor_future is not None and self.executor_future.running():
             self.cancel.set()
-            self.pool.shutdown()
+        self.pool.shutdown()
+        self.logger.info("Worker[%s] is exiting due to ActorExitRequest.", str(self.worker_id))
 
     def receiveMsg_BenchmarkFailure(self, msg, sender):
         # sent by our no_retry infrastructure; forward to master