Use cURL multi interface in HTTPSFuture

Summary:
This violates the contract of `isReady()` which shouldn't block.
My implementation waits for any available handle.
The reason is that cURL doesn't expose the file descriptors for select: http://svn.php.net/viewvc/php/php-src/tags/php_5_4_0/ext/curl/multi.c?view=markup#l193
This will cause waiting for HTTPS future even if non-HTTPS future is already available.
I think that it is not a big deal.

Other solution is to rewrite `FutureIterator` to call Future-specific waiters instead of `Future::waitForSockets()`.

Other solution is to remove `curl_multi_select()` and always busy wait.

Test Plan:
  $futures = array();
  for ($i=0; $i < 5; $i++) {
    $futures[] = new HTTPSFuture('http://www.vrana.cz/sleep.php');
  }

  foreach (Futures($futures) as $future) {
    list(, $content) = $future->resolve();
    echo "$content\n";
  }

Please suggest me how to benchmark it (some Arcanist command?).

Reviewers: vrana, btrahan

Reviewed By: btrahan

CC: aran, Korvin, edward

Maniphest Tasks: T1683

Differential Revision: https://secure.phabricator.com/D2471