It's impossible to upgrade connection asynchroniously

[kekekeks]

If the response wasn't sent inside RequestHandler::onRequest and is sent later, RequestHandler will recieve EOM instead of upgrade.

I. e. this scenario does not work as expected:

void EchoHandler::onRequest(std::unique_ptr<HTTPMessage> headers) noexcept {
    auto ev = folly::EventBaseManager::get()->getExistingEventBase();
    std::thread([=](){
        //Do some logic here
        ev->runInEventBaseThread([=](){
            auto builder = make_unique<ResponseBuilder>(downstream_);
            builder->acceptUpgradeRequest(ResponseBuilder::UpgradeType::HTTP_UPGRADE, "websocket");
        });
    }).detach();
}

[kekekeks]

Oh, sorry. I figured out, that I should call pauseIngress/resumeIngress to do that.