MQTT client keep alive fixes

- Move keep alive management up to the Mongoose library
 - Only outgoing control messgaes should reset keepalive timer
 - Add unit test

https://forum.mongoose-os.com/discussion/1155/mqtt-keep-alive-compliance-issue

PUBLISHED_FROM=f86e30744ded53a6f7f96afec066b4ff3b4372c0

mongoose.h

@@ -5105,6 +5105,7 @@ struct mg_send_mqtt_handshake_opts {
 /* mg_mqtt_proto_data should be in header to allow external access to it */
 struct mg_mqtt_proto_data {
   uint16_t keep_alive;
+  double last_control_time;
 };
 
 /* Message types */