Support for casablanca (cpp rest sdk) version 2.8

[anton-kvant]

Hello.
Why does odatacpp-server is compatible only with Casabanca version 2.1. Casablanca version 2.8 is already available and it has good features: HTTPS, Oauth support.

After updating libcpprest from v2.1.0 to v2.8.0 I've noticed ~ 40 ms delay in Casablanca's PPLX tasks.

 void handlePost(web::http::http_request request) {
      // delay in next line
      std::string body = request.extract_string().get();
      web::http::http_response response(web::http::status_codes::OK);
      response.set_body(body);

      request.reply(response)
          .then(std::bind(&TestServer::handleError, std::placeholders::_1));
      return;
   }

When there is no OData in my code, the same code executed fast (delay about 0.05 ms)