Fix problems with QueryFuture in HPHP

Summary:
fb_parallel_query() has a weird semantics for indexing results.
Also, it doesn't support queries with different columns.
mysql_fetch_result() raises warning if there's an error.

Test Plan:

$futures = array(
  new QueryFuture($conn, 'SELECT 1'),
  new QueryFuture($conn, 'SELECT 2 FROM x'),
  new QueryFuture($conn, 'DO 3'),
);
foreach (Futures($futures) as $future) {
  try {
    $future->resolve();
    print_r($future->resolve());
  } catch (Exception $ex) {
    echo $ex->getMessage()."\n";
  }
}

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5057