Allow setting multi-request concurrency

[LeMoussel]

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);

[rmccue]

This isn't currently supported, but would make sense to do so.

[soulseekah]

For cURL we should be using curl_multi_select and adding in new handles as the others are freed. This should be fairly straightforward to implement, but hard to test. I'd like to give it a shot.

Any ideas, preferences on naming the option? concurrent, threads (not technically), limit?

For fsockopen stream_select would actually work, but we need to rewrite the request_multiple in a better way, split out the read and parse parts. Definitely possible but out of scope of this issue, since fsockopen request_multiple is just a series wrapper around the regular request. TBD in #317

Possibly related #53