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

docs/c-api/mqtt.h/struct_mg_mqtt_proto_data.md

@@ -5,6 +5,7 @@ symbol_kind: "struct"
 signature: |
   struct mg_mqtt_proto_data {
     uint16_t keep_alive;
+    double last_control_time;
   };
 ---