Remove "identifier" field be from JSON form of DataSegment, or clarify it's purpose

http://druid.io/docs/latest/dependencies/metadata-storage says:

> The payload column stores a JSON blob that has all of the metadata for the segment (some of the data stored in this payload is redundant with some of the columns in the table, that is intentional).

I think a field should be either removed or "that is intentional" should be replaced with more elaborate explanation.

Start with the metadata-storage documentation at http://druid.io/docs/latest/dependencies/metadata-storage and inspect the description of the payload column and DataSegment JSON. Determine whether the identifier field is redundant or needs a clearer explanation; done means the documentation makes the field's purpose unambiguous or no longer includes it.

Written by the indexing model from the issue text.