Update secondary index responses

2i and CS bucket fold responses have additional fields from the previous 2i implementation. 2i now has a `return_terms` option, which will change the response, and both 2i and CS bucket fold have a `continuation` binary that will be returned (or `undefined`). This change does a couple things:

    * puts the continuation into the `done` message for streaming responses
    * encapsulates 2i buffered and streaming responses into a record
    * changes the response tuple-wrapper for CS bucket fold from `objects`
      to `ok`.
    * adds new types for the continuation and new records