Fixes WordPress#283 : Fixes CPU busy loop when using request_multiple.

This will require more testing across various libcurl versions.
I doubt that the timeout is necessary for curl_multi_select
(calls select() if it can),
but leaving in one just in case of bugs, so that it will end.
- Haven't thoroughly checked for relevant libcurl bugs.

Asynchronously wait for events with a short timeout if CURLM_CALL_MULTI_PERFORM

library/Requests/Transport/cURL.php

@@ -219,8 +219,20 @@ public function request_multiple($requests, $options) {
 
 			do {
 				$status = curl_multi_exec($multihandle, $active);
+				// Return immediately if there's work to be done.
+				if ($status !== CURLM_CALL_MULTI_PERFORM) {
+					break;
+				}
+				// Block until there is activity on any of the curl_multi connections.
+				// Timeouts can be specified in microseconds.
+				// Provide curl_multi_select with a timeout of 100ms (May not be necessary).
+				// As a fallback, if curl_multi_select couldn't properly asynchronously wait for input, sleep for 0.1 ms.
+				$select_status = curl_multi_select($multihandle, 0.1);
+				if ($select_status === -1) {
+					usleep(100);
+				}
 			}
-			while ($status === CURLM_CALL_MULTI_PERFORM);
+			while (true);
 
 			$to_process = array();