Use Lisk sets in fact update iterator

Summary:
Fact engines loading dependent objects are super slow because they load them one by one.
This diff put each page in a Lisk set allowing engines to use `loadRelatives()`.

It also introduces `clearSet()` method which is somewhat neccessary in PHP < 5.3 or with disabled cyclic [[ http://php.net/gc | GC ]].

Test Plan:
  $iterator = new PhabricatorFactUpdateIterator(new DifferentialRevision());
  foreach ($iterator as $revision) {
    $diffs = $revision->loadRelatives(new DifferentialDiff(), 'revisionID');
    echo memory_get_usage() . "\n";
  }

Experienced not-steadily-increasing memory usage and much faster loading.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3247

src/applications/fact/extract/PhabricatorFactUpdateIterator.php

@@ -28,8 +28,11 @@ final class PhabricatorFactUpdateIterator extends PhutilBufferedIterator {
   private $position;
   private $ignoreUpdatesDuration = 15;
 
+  private $set;
+
   public function __construct(LiskDAO $object) {
-    $this->object = $object;
+    $this->set = new LiskDAOSet();
+    $this->object = $object->putInSet($this->set);
     $this->position = '0:0';
   }
 
@@ -53,6 +56,8 @@ final class PhabricatorFactUpdateIterator extends PhutilBufferedIterator {
   protected function loadPage() {
     list($after_epoch, $after_id) = explode(':', $this->cursor);
 
+    $this->set->clearSet();
+
     // NOTE: We ignore recent updates because once we process an update we'll
     // never process rows behind it again. We need to read only rows which
     // we're sure no new rows will be inserted behind. If we read a row that

src/infrastructure/storage/lisk/LiskDAOSet.php

@@ -39,6 +39,7 @@
 final class LiskDAOSet {
   private $daos = array();
   private $relatives = array();
+  private $subsets = array();
 
   public function addToSet(LiskDAO $dao) {
     $this->daos[] = $dao;
@@ -46,6 +47,20 @@ final class LiskDAOSet {
     return $this;
   }
 
+  /**
+   * The main purpose of this method is to break cyclic dependency.
+   * It removes all objects from this set and all subsets created by it.
+   */
+  final public function clearSet() {
+    $this->daos = array();
+    $this->relatives = array();
+    foreach ($this->subsets as $set) {
+      $set->clearSet();
+    }
+    return $this;
+  }
+
+
   /**
    * See @{method:LiskDAO::loadRelatives}.
    */
@@ -70,6 +85,7 @@ final class LiskDAOSet {
         $relatives = array();
       } else {
         $set = new LiskDAOSet();
+        $this->subsets[] = $set;
         $relatives = $object->putInSet($set)->loadAllWhere(
           '%C IN (%Ls) %Q',
           $foreign_column,