R1:f841491524f1

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…