Fix profiling Conduit calls

Summary: This error was masked by resolving HTTPS futures in serial.

Test Plan:

PhutilServiceProfiler::getInstance()->enableDiscardMode();
$conduit = new ConduitClient('https://secure.phabricator.com/');
$futures = array(
  $conduit->callMethod('conduit.ping', array()),
  $conduit->callMethod('conduit.ping', array()),
);

Futures($futures)->resolveAll();

Didn't see:

Notice: Undefined offset: 2 in libphutil/src/serviceprofiler/PhutilServiceProfiler.php on line 42

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5027