MB-10849: Adding bucket priority to cbstats

Change-Id: I6d44598688a85a5ae46d795791ee6eef87ea6051
Reviewed-on: http://review.couchbase.org/35908
Reviewed-by: Sundararaman Sridharan <sundar@couchbase.com>
Tested-by: Sundararaman Sridharan <sundar@couchbase.com>