Revert "replace native implementation of curl_multi_await with a systemlib one"

[fredemmott]

fixes #8156
refs #7485
refs #8043

The previous stability issues with the native impl were likely not
actually an issue with async curl, but with async more generally with
libevent2.

93c31ec appears to have fixed the original stability issues (and the issues with
stream_await()); it just looked like curl_multi_await() and
stream_await() were buggy as:

• FB rarely uses these functions (in particular, for our use case, async
  curl is generaly less performant than sync curl - but that depends on
  what other load you have, and how long requests take)
• FB uses libevent1
• curl_multi_await() and stream_await() are among the most useful
  external features
  ... so, an HHVM issue when using libevent2 looks like a curl + stream
  async issue

Test Plan: ran test programs from previous curl issues

Crashes or gets stuck (doesn't on master or with this diff):

<?hh

async function get_curl_content(Set<string> $urls): Awaitable<Vector<string>> {

  $chs = Vector {};
  foreach ($urls as $url) {
    $ch = curl_init($url);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
    $chs[] = $ch;
  }

  $mh = curl_multi_init();
  foreach ($chs as $ch) {
    curl_multi_add_handle($mh, $ch);
  }

  $active = -1;
  do {
    $ret = curl_multi_exec($mh, &$active);
  } while ($ret == CURLM_CALL_MULTI_PERFORM);

  while ($active && $ret == CURLM_OK) {
    $select = await curl_multi_await($mh);
    if ($select === -1) {
      // https://bugs.php.net/bug.php?id=61141
      await \HH\Asio\usleep(100);
    }
    do {
      $ret = curl_multi_exec($mh, &$active);
    } while ($ret == CURLM_CALL_MULTI_PERFORM);
  }

  $content = Vector {};

  foreach ($chs as $ch) {
    $str = (string) curl_multi_getcontent($ch);
    $content[] = substr($str, 0, 10);
    curl_multi_remove_handle($mh, $ch);
  }

  curl_multi_close($mh);

  return $content;
}

function run(): void {
  $urls = Set {};
  for($i=1;$i<10;$i++) $urls->add('https://hhvm.com/?test='.$i);
  $content = \HH\Asio\join(get_curl_content($urls));
  var_dump($content);
}

while (true) {
    run();
}

Spins 100% CPU (does on master, doesn't with this diff):

<?hh

$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, "https://httpbin.org/delay/10");

// this line takes 100% cpu for 10 seconds
\HH\Asio\join(\HH\Asio\curl_exec($ch));

// this line *does not* takes 100% cpu for 10 seconds
// curl_exec($ch);

curl_close($ch);

[hhvm-bot]

fredemmott has imported this pull request. If you are a Facebook employee, you can view this diff on Phabricator.