implemented ns_config:compute_global_rev/1

Change-Id: I373966d12d1918dbd87010a3e6e0d26ecf2a3032
Reviewed-on: http://review.couchbase.org/44453
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>