On MQTTClient.c function mqtt_subscribe, if more than MQTT_MAX_MESSAGE_HANDLERS are registered, the variable rc receives the value of grantedQoS.
If grantedQoS = 0, and all handlers are already occupied, it goes on and returns 0 to the caller.
* Added a MQTT pub/sub example of using AWS IoT (via ECC based TLS1.2 connection).
* Fixed a buffer overflow issue when receiving large MQTT packet.
* Reset TLS connection on read/write errors.
