Add logs around usage tracker shutdown (#3391)

Parsl shutdown logs are deliberately rich at shutdown time because this is an
incredibly hang prone part of the code-base.
See https://github.com/Parsl/parsl/labels/safe-exit

This PR adds logs bracketing the shutdown of the usage tracker, in line
with later shutdowns of the job status poller, executors and monitoring.

parsl/dataflow/dflow.py

@@ -1245,8 +1245,10 @@ def cleanup(self) -> None:
                 self._checkpoint_timer.close()
 
         # Send final stats
+        logger.info("Sending end message for usage tracking")
         self.usage_tracker.send_end_message()
         self.usage_tracker.close()
+        logger.info("Closed usage tracking")
 
         logger.info("Closing job status poller")
         self.job_status_poller.close()