MB-100: introduced ns_bucket:get_bucket_with_vclock

Change-Id: Ia7ade2970c4661b56f2f084c014f3b8f7a10ff6c
Reviewed-on: http://review.couchbase.org/30322
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>