Stop passing around _type and _id in document source

[MatMoore]

From https://trello.com/c/y4ckV2Qt/18-stop-passing-around-type-and-id-in-document-source

The _type and _id fields have been removed from the document source in PR 824. The Rummager code still adds them to the document hash as a convenient (but confusing) way of passing the type and id around with the rest of the document.

Example in amender.rb, we still add _type and _id to the document _source:

      # For backwards-compatibility, ensure that the source _type and _id are
      # the same as the main Elasticsearch _type and _id
      document_source["_type"] = raw_document["_type"]
      document_source["_id"] = raw_document["_id"]

Can we separate _type and _id from the rest of the document fields so they don't look like fields which will be stored in the source in Elasticsearch?