prevented direct vbucket access via capi in few places

We had _changes and even some bits of CRUD against vbucket still
exposed.

Change-Id: If7261aa011948f135ba3119348427e9b7af4a549
Reviewed-on: http://review.couchbase.org/39512
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>