Extend LiskMigrationIterator from PhutilBufferedIterator

Summary: PhutilBufferedIterator now implements all the nonspecific logic here.

Test Plan:
Created a test script like this:

$iterator = new LiskMigrationIterator(new DifferentialRevision());
$iterator->setPageSize(3);

foreach ($iterator as $key => $rev) {
  echo "{$key}: ".$rev->getID()."\n";
}

Ran it and verified sensible iteration results.

Reviewers: vrana, btrahan

Reviewed By: vrana

CC: aran

Maniphest Tasks: T1562

Differential Revision: https://secure.phabricator.com/D3076