Introduce QueryFuture

Summary:
This supports four ways for executing queries against MySQL:

1. Async queries
2. Parallel queries
3. Multi queries: PHP, HPHP
4. Plain old single queries

This is just a rough concept, I'll improve it later.

Test Plan:

$result = id(new QueryFuture($conn, 'SELECT 1'))->resolve();
print_r($result);

$futures = array(
  new QueryFuture($conn, 'SELECT 1'),
  new QueryFuture($conn, 'SELECT 2'),
);
foreach (Futures($futures) as $future) {
  print_r($future->resolve());
}

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5025