fix: added flush() for proper shutdown

https://telecominfraproject.atlassian.net/browse/WIFI-13597

NOTE: This PR is port of https://github.com/Telecominfraproject/wlan-cloud-ucentralgw/pull/361

Signed-off-by: Ivan Chvets <ivan.chvets@kinarasystems.com>

src/framework/KafkaManager.cpp

@@ -123,6 +123,7 @@ namespace OpenWifi {
 			}
 			Note = Queue_.waitDequeueNotification();
 		}
+		Producer.flush();
 		poco_information(Logger_, "Stopped...");
 	}