Add RBAC skeleton to memcached

As the overall specification evolves we'll adapt the changes
(and add proper documentation).

Change-Id: Id751355bd614d5384577f4079a2ed74a3635d440
Reviewed-on: http://review.couchbase.org/41182
Tested-by: Trond Norbye <trond.norbye@gmail.com>
Reviewed-by: Dave Rigby <daver@couchbase.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>