CCBC-508: Mark hashkey as volatile in the API

This commit marks the hashkey/nhashkey fields in the v2 API as being
private and volatile.

The APIv3 hashkey field has been reworded to _hashkey to further convey
the private/volatile nature of this key.

Change-Id: Ib0d21e688aeb9638250b769e9c29ee0f47dde682
Reviewed-on: http://review.couchbase.org/41317
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>