Event-based discovery `START_SYNC` is actually polling-based

[cmaglie]

As it seems from the START_SYNC implementation:

        std::thread([&] {
          j["message"] = "OK";
          std::cout << j.dump(2) << std::endl;
          while (state != QUIT) {
            if (state == START_SYNC) {
              probe_devices(ctx);
              print_event();
            }
            std::this_thread::sleep_for(std::chrono::seconds(5));
          }
        }).detach();

It does a poll every 5 seconds, this means that events may be delayed up to 5 seconds.
Also, polling is not ideal for discoveries, we must find a way to make it event-based to deliver events in real time like all other discoveries.