NCBC-586: Make AddRef and Release on IBucket not public

This commit adds an internal interface IRefCountable with AddRef and
Release members, so that the two methods are not publically callable on an
IBucket implementation.

Change-Id: I11f628094aa1f65ff4780f42e00bbfb088e78898
Reviewed-on: http://review.couchbase.org/41354
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Jeffry Morris <jeffrymorris@gmail.com>