Reset Bulk operations while a new Bulk transaction is called.

Batches can have a limited memory. 
This hack allows requesting the bulk API several times with the same PHP object. 
```php
while($continue) {
  $elastic->begin();
  $rows = $pdo->query($query);
  foreach ($rows as $row)
    $elastic->index($row);
  $elastic->commit();
}
```