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