Support for _parent & _routing in bulk index/delete operations

This adds support for the following special keys for bulk index/delete operations: `_routing`, `_percolate`, `_parent`, `_timestamp`, and `_ttl`, in addition of `_index`, `_type` and `_id` that are already supported. I needed `_routing` and `_parent` to migrate an index with parent/child relationships, and I added a few more in this PR.