Extend support for attachment-related query params

Until now, the boolean query parameters `attachments` and
`att_encoding_info` have only been supported for the document API
endpoint (`/{db}/{docid}`).

This extends support for queries to the changes (`/{db}/_changes`) and
view (`/{db}/_design/{ddoc}/_view/{view}`) API endpoints:

* If `include_docs` and `attachments` equal `true`, the Base64-encoded
  contents of attachments are included with the documents in changes or
  view query results, respectively.

* If `include_docs` and `att_encoding_info` equal `true`, encoding
  information is included in attachment stubs if the particular
  attachment is compressed.

Closes COUCHDB-1923.