Cancel all command when MQTT command failed (#814)

* Use MQTT Agent provided API, MQTTAgent_CancelAll, to clear queue when
  MQTT command failed.

FreeRTOS-Plus/Demo/AWS/Ota_Windows_Simulator/Ota_Over_Http_Demo/DemoTasks/OtaOverHttpDemoExample.c

@@ -2132,7 +2132,7 @@ static void prvMQTTAgentTask(void* pParam)
         xMQTTStatus = MQTTAgent_CommandLoop(&xGlobalMqttAgentContext);
 
         /* Clear Agent queue so that no any pending MQTT operations are processed. */
-        xQueueReset(xCommandQueue.queue);
+        MQTTAgent_CancelAll(&xGlobalMqttAgentContext);
 
         /* Success is returned for application initiated disconnect or termination. The socket will also be disconnected by the caller. */
         if (xMQTTStatus != MQTTSuccess)

FreeRTOS-Plus/Demo/AWS/Ota_Windows_Simulator/Ota_Over_Mqtt_Demo/DemoTasks/OtaOverMqttDemoExample.c

@@ -1725,7 +1725,7 @@ static void prvMQTTAgentTask(void* pParam)
         xMQTTStatus = MQTTAgent_CommandLoop(&xGlobalMqttAgentContext);
 
         /* Clear Agent queue so that no any pending MQTT operations are processed. */
-        xQueueReset(xCommandQueue.queue);
+        MQTTAgent_CancelAll(&xGlobalMqttAgentContext);
 
         /* Success is returned for application initiated disconnect or termination. The socket will also be disconnected by the caller. */
         if (xMQTTStatus != MQTTSuccess)