Allow cursors to add KVs to the find response
Allow cursors to add KVs to the find response

This change allows cursors to set top level key/value pairs in the
response to _find requests. This is ground work for Search based indexes
that will need to set a bookmark value for clients.

BugzId: 33294

fkaempfer/couchdb,fkaempfer/couchdb,cloudant/mango,apache/couchdb-mango,fkaempfer/couchdb,kxepal/couchdb-mango,fkaempfer/couchdb,fkaempfer/couchdb,cloudant/couchdb-mango,fkaempfer/couchdb