input elasticsearch: optionally capture document metadata (document type...

..., index, document id)
The patch also contains some debug and failure handling for the previous fix  https://github.com/logstash/logstash/pull/670 on the scroll id
I'm going to rename this to "docinfo" and collapse the settings into a single setting:

`config :docinfo, :validate => :string`

When nil, no document info is saved. When set, is set to the field name.