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

src/__phutil_library_map__.php

@@ -4032,6 +4032,7 @@ phutil_register_library_map(array(
     'PhabricatorPygmentSetupCheck' => 'applications/config/check/PhabricatorPygmentSetupCheck.php',
     'PhabricatorQuery' => 'infrastructure/query/PhabricatorQuery.php',
     'PhabricatorQueryConstraint' => 'infrastructure/query/constraint/PhabricatorQueryConstraint.php',
+    'PhabricatorQueryIterator' => 'infrastructure/storage/lisk/PhabricatorQueryIterator.php',
     'PhabricatorQueryOrderItem' => 'infrastructure/query/order/PhabricatorQueryOrderItem.php',
     'PhabricatorQueryOrderTestCase' => 'infrastructure/query/order/__tests__/PhabricatorQueryOrderTestCase.php',
     'PhabricatorQueryOrderVector' => 'infrastructure/query/order/PhabricatorQueryOrderVector.php',
@@ -9876,6 +9877,7 @@ phutil_register_library_map(array(
     'PhabricatorPygmentSetupCheck' => 'PhabricatorSetupCheck',
     'PhabricatorQuery' => 'Phobject',
     'PhabricatorQueryConstraint' => 'Phobject',
+    'PhabricatorQueryIterator' => 'PhutilBufferedIterator',
     'PhabricatorQueryOrderItem' => 'Phobject',
     'PhabricatorQueryOrderTestCase' => 'PhabricatorTestCase',
     'PhabricatorQueryOrderVector' => array(

src/infrastructure/storage/lisk/PhabricatorQueryIterator.php

@@ -0,0 +1,41 @@
+<?php
+
+final class PhabricatorQueryIterator extends PhutilBufferedIterator {
+
+  private $query;
+  private $pager;
+
+  public function __construct(PhabricatorCursorPagedPolicyAwareQuery $query) {
+    $this->query = $query;
+  }
+
+  protected function didRewind() {
+    $this->pager = new AphrontCursorPagerView();
+  }
+
+  public function key() {
+    return $this->current()->getID();
+  }
+
+  protected function loadPage() {
+    if (!$this->pager) {
+      return array();
+    }
+
+    $pager = clone $this->pager;
+    $query = clone $this->query;
+
+    $results = $query->executeWithCursorPager($pager);
+
+    // If we got less than a full page of results, this was the last set of
+    // results. Throw away the pager so we end iteration.
+    if (count($results) < $pager->getPageSize()) {
+      $this->pager = null;
+    } else {
+      $this->pager->setAfterID($pager->getNextPageID());
+    }
+
+    return $results;
+  }
+
+}