Command acknowledgement deadlock inside the publish loop

[nik-markovic]

When command is received while telemetry is being published, this deadlock could occur:

• iotc_nrf_mqtt_publish(telemetry) [ sys_mutex_lock(&mutex_mqtt_pub, K_FOREVER); ]
• iotc_nrf_mqtt_loop();
• mqtt_handle_rx()
• event_notify()
• mqtt_evt_handler(MQTT_EVT_PUBLISH)
• config->data_cb()
• iotc_on_mqtt_data()
• iotcl_process_event()
• iotc_process_callback()
• on_command()
• iotconnect_sdk_send_packet(ack) [ attempts to get lock which is not released]

Somewhere this ends up releasing with message "get_event_payload: EAGAIN", but only after a long time.

I am also concerned about calling iotc_nrf_mqtt_loop() inside iotc_nrf_mqtt_publish(). We may end up using up too much stack because this could also result in an inbound mqtt message from broker. iotc_nrf_mqtt_loop() should probably only be called from main to be able to better estimate stack usage. This could maybe even cause recursion. I am not sure how much of an impact this change would have on MQTT confirmation code.

@syjen Assigning to you, but feel free to reassign this ticket to someone else on your team.