Make it immutable

[mpyw]

Current implementions are based on raw Symfony/BrowserKit, however, they are too much stateful. Concurrent requests may break their logics. So I will adopt the new model.

Something like:

$ccr = new ClientCrawlerResponse();
$new_ccr = yield $ccr->requestAsync('GET', $url); 

Combine Client, Crawler and Response to make them immutable.