Clarify in log that a HTEX message is a heartbeat *response* (#3990)

## Type of change

- Update to human readable text: Documentation/error messages/comments

Author: benclifford

parsl/executors/high_throughput/process_worker_pool.py

@@ -379,7 +379,7 @@ def interchange_communicator(self, pair_setup: threading.Event):
                 last_interchange_contact = time.time()
 
                 if tasks == HEARTBEAT_CODE:
-                    logger.debug("Got heartbeat from interchange")
+                    logger.debug("Got heartbeat response from interchange")
                 elif tasks == DRAINED_CODE:
                     logger.info("Got fully drained message from interchange - setting kill flag")
                     self._stop_event.set()