Add LiskRawMigrationIterator

Summary: Ref T2222. I need this to migrate the Differential comment tables, because they are large. We have the similar `LiskMigrationIterator` already, but it won't work here because I intend to destroy the original objects after migrating them.

Test Plan:
Wrote a script to iterate over the `differential_comment` table, got reasonable output:

{P869}

Reviewers: btrahan

Reviewed By: btrahan

CC: chad, aran

Maniphest Tasks: T2222

Differential Revision: https://secure.phabricator.com/D6264

src/__phutil_library_map__.php

@@ -626,6 +626,7 @@ phutil_register_library_map(array(
     'LiskIsolationTestDAO' => 'infrastructure/storage/lisk/__tests__/LiskIsolationTestDAO.php',
     'LiskIsolationTestDAOException' => 'infrastructure/storage/lisk/__tests__/LiskIsolationTestDAOException.php',
     'LiskMigrationIterator' => 'infrastructure/storage/lisk/LiskMigrationIterator.php',
+    'LiskRawMigrationIterator' => 'infrastructure/storage/lisk/LiskRawMigrationIterator.php',
     'ManiphestAction' => 'applications/maniphest/constants/ManiphestAction.php',
     'ManiphestAuxiliaryFieldDefaultSpecification' => 'applications/maniphest/auxiliaryfield/ManiphestAuxiliaryFieldDefaultSpecification.php',
     'ManiphestAuxiliaryFieldSpecification' => 'applications/maniphest/auxiliaryfield/ManiphestAuxiliaryFieldSpecification.php',
@@ -2476,6 +2477,7 @@ phutil_register_library_map(array(
     'LiskIsolationTestDAO' => 'LiskDAO',
     'LiskIsolationTestDAOException' => 'Exception',
     'LiskMigrationIterator' => 'PhutilBufferedIterator',
+    'LiskRawMigrationIterator' => 'PhutilBufferedIterator',
     'ManiphestAction' => 'ManiphestConstants',
     'ManiphestAuxiliaryFieldDefaultSpecification' => 'ManiphestAuxiliaryFieldSpecification',
     'ManiphestAuxiliaryFieldSpecification' => 'PhabricatorMarkupInterface',

src/infrastructure/storage/lisk/LiskRawMigrationIterator.php

@@ -0,0 +1,39 @@
+<?php
+
+final class LiskRawMigrationIterator extends PhutilBufferedIterator {
+
+  private $conn;
+  private $table;
+  private $cursor;
+  private $column = 'id';
+
+  public function __construct(AphrontDatabaseConnection $conn, $table) {
+    $this->conn = $conn;
+    $this->table = $table;
+  }
+
+  protected function didRewind() {
+    $this->cursor = 0;
+  }
+
+  public function key() {
+    return idx($this->current(), $this->column);
+  }
+
+  protected function loadPage() {
+    $page = queryfx_all(
+      $this->conn,
+      'SELECT * FROM %T WHERE %C > %d ORDER BY ID ASC LIMIT %d',
+      $this->table,
+      $this->column,
+      $this->cursor,
+      $this->getPageSize());
+
+    if ($page) {
+      $this->cursor = idx(last($page), $this->column);
+    }
+
+    return $page;
+  }
+
+}