Add PhabricatorQueryIterator, for buffered iteration over a CursorPagedPolicyAwareQuery

Summary:
See D19446. This should make it easier to process larger, more complex result sets in constant memory.

Today, `LiskMigrationIterator` takes constant memory but can't apply `needX()` reqeusts or `withY(...)` constraints.

Using a raw `Query` can handle this stuff, but requires memory proportional to the size of the result set.

Offer the best of both worlds: constant memory and full access to the power of `Query` classes.

Test Plan:
Used this script to iterate over every commit, saw sensible behavior:

```name=list-commits.php
<?php

require_once 'scripts/init/init-script.php';

$viewer = PhabricatorUser::getOmnipotentUser();

$query = id(new DiffusionCommitQuery())
->setViewer($viewer);

$iterator = new PhabricatorQueryIterator($query);
foreach ($iterator as $commit) {
echo $commit->getID()."\n";
}
```

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D19450