Allow setting multi-request concurrency

With request_multipleis it possible to send multiple requests simultaneously.
Is it possible to set the limit for how many requests will be execute simultaneously?

Because if it's too high, requests are likely to fail more frequently or automated software may perceive as a DOS attack and automatically block further requests.

For example I have an array with 1000 urls

$requests = array( 1000 * array( 'url' => 'http://urlofsitetotest.com/), );
// Setup a callback
function my_callback(&$request, $id) {
	var_dump($id, $request);
}
// Tell Requests to use the callback
$options = array('complete' => 'my_callback',)
// Set 5 requests will be execute simultaneously
// Requests::setSimultaneousLimit(5) => Does option exists ?
// Send the request!
$responses = Requests::request_multiple($requests, $options);

Start at Requests::request_multiple and trace how requests are scheduled and callbacks are handled. Determine where a configurable concurrency limit fits, then verify that the limit caps simultaneous requests while preserving the existing callback behavior.