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.