kind of async support

I would like to propose an additional async like approach for Request. The client API would look something like this:

// trigger the requests
$future = Requests::request_async($requests, $options);

// do whatever PHP stuff you like
echo "lala";

// ...
// after you have done everything which is not related to the asyncRequest

 // wait for all curl requests which are not finished yet and 
// returns their results as request_multiple would do.
// also frees all remaining resources
$results = $future->getResult();

// process your result

to implement this kind of handling I would like to split Requests_Transport_cURL::request_multiple() in two smaller parts.

The first request_async part would start the requests (curl_init, curl_setopt, curl_multi_init, curl_multi_add_handle and kick of the requests using curl_multi_exec).

The second part $future->getResult() will run the loop over curl_multi_exec until everything is finished, cleanup using curl_multi_remove_handle & curl_multi_close.

this kind of request handling is AFAIK only possible using curl, therefore the default implementation would do the stuff synchrously (call request_mulitple).

I am willing to provide the implementation in case you give your 👍.

Start by reading Requests_Transport_cURL::request_multiple() and the Requests::request_async() API described in the issue. Define how the future object starts and completes curl work, releases remaining handles, and returns the same results as request_multiple(); verify that non-cURL transports retain synchronous behavior.