fix: modified kafka manager to use poll in producer

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

NOTE: This fix is port of https://github.com/Telecominfraproject/wlan-cloud-ucentralgw/pull/360

Summary of changes:
- Modified code in KafkaManager to use poll instead of flush for every
  messages sent. flush is used only on empty internal notification queue
  in idle times.

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

src/framework/KafkaManager.cpp

@@ -107,7 +107,7 @@ namespace OpenWifi {
 					NewMessage.partition(0);
 					NewMessage.payload(Msg->Payload());
 					Producer.produce(NewMessage);
-					Producer.flush();
+					Producer.poll((std::chrono::milliseconds) 0);
 				}
 			} catch (const cppkafka::HandleException &E) {
 				poco_warning(Logger_,
@@ -117,6 +117,10 @@ namespace OpenWifi {
 			} catch (...) {
 				poco_error(Logger_, "std::exception");
 			}
+			if (Queue_.size() == 0) {
+				// message queue is empty, flush all previously sent messages
+				Producer.flush();
+			}
 			Note = Queue_.waitDequeueNotification();
 		}
 		poco_information(Logger_, "Stopped...");