R1:c614af53bfe9

Add PhutilFactsUpateIterator

Summary: This iterator processes objects that have been updated.

Test Plan:
Ran this test script:

$cursor = null;
$table = new DifferentialRevision();
while (true) {
$iterator = new PhabricatorFactsUpdateIterator($table, $cursor);
foreach ($iterator as $new_cursor => $update) {
echo "{$new_cursor} => D".$update->getID()."\n";
$cursor = $new_cursor;
}
echo "Zzz...\n";
sleep(5);
}

Verified it iterated over every object and then stopped.