MB-8726: added vclock:count_changes

Change-Id: Ic44479049d0484a44a1945614c6f3e878b4766c5
Reviewed-on: http://review.couchbase.org/29998
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>