MB-8726: added rev field to terse bucket details

rev field can be used to compare bucket details from different nodes
for freshness.

vclock _of all buckets_ is used to compute that. Simply because we
don't have vclock per bucket.

That makes rev field a bit weird. Same bucket details may have
different rev depending on when they where cached. But what really
matters is that fresher bucket details will always have greater rev
field. Which is what we really need.

Change-Id: Iad5ec27d40e482501f91bb690b6c9c80948a77ad
Reviewed-on: http://review.couchbase.org/30000
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>