SERVER-14833 {_id:-1} index builds should no-op if _id index exists
SERVER-14833 Fix issue where 98eb2f1d broke text index spec rewrite
SERVER-12209 _id prohibitions

Prohibit _id of type Array or Undefined.
Also, if _id is an Object, recursively check the object for $fieldnames.
SERVER-14833 fts_index3.js build correct index, minor comment fixes