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

See #284 for background information

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
fails. Use shorter timeouts when the total time elapsed so far is
shorter. Make the largest possible manual usleep 2ms.

library/Requests/Transport/cURL.php

@@ -214,14 +214,44 @@ public function request_multiple($requests, $options) {
 
 		$request['options']['hooks']->dispatch('curl.before_multi_exec', array(&$multihandle));
 
+		$made_progress = true;
+		$all_requests_start = microtime(true);
+		$microseconds_taken_by_last_curl_operation = 0;
 		do {
 			$active = false;
 
+			if (!$made_progress) {
+				// For a small fraction of slow requests, curl_multi_select will busy loop and return immediately with no indication of error (it returns 0 immediately with/without setting curl_multi_errno).
+				// (https://github.com/rmccue/Requests/pull/284)
+				// To mitigate this, add our own sleep if curl returned but no requests completed (failing or succeeding)
+				// - The amount of time to sleep is the smaller of 1ms or 10% of total time spent waiting for curl requests
+				//   (10% was arbitrarily chosen to slowing down requests that would normally take less than 1 millisecond)
+				// - The amount of time that was already spent in curl_multi_exec+curl_multi_select in the previous request is subtracted from that.
+				//   (E.g. if curl_multi_select already slept for 2 milliseconds, curl_multi_select might be operating normally)
+				$elapsed_microseconds = (microtime(true) - $all_requests_start) * 1000000;
+				$microseconds_to_sleep = min($elapsed_microseconds / 10, 2000) - $microseconds_taken_by_last_curl_operation;
+				if ($microseconds_to_sleep >= 1) {
+					usleep($microseconds_to_sleep);
+				}
+			}
+			$operation_start = microtime(true);
+			$made_progress = false;
+			$is_first_multi_exec = true;
 			do {
+				if (!$is_first_multi_exec) {
+					// Sleep for 1 millisecond to avoid a busy loop that would chew CPU
+					// See ORA2PG-498
+					usleep(1000);
+				}
 				$status = curl_multi_exec($multihandle, $active);
+				$is_first_multi_exec = false;
 			}
 			while ($status === CURLM_CALL_MULTI_PERFORM);
 
+			// curl_multi_select will sleep for at most 50 milliseconds before returning.
+			curl_multi_select($multihandle, 0.05);
+			$microseconds_taken_by_last_curl_operation = (microtime(true) - $operation_start) * 100000;
+
 			$to_process = array();
 
 			// Read the information as needed
@@ -234,6 +264,7 @@ public function request_multiple($requests, $options) {
 
 			// Parse the finished requests before we start getting the new ones
 			foreach ($to_process as $key => $done) {
+				$made_progress = true;
 				$options = $requests[$key]['options'];
 				if (CURLE_OK !== $done['result']) {
 					//get error string for handle.