MB-7435: Account for chk start in numOpenChkItems

Change-Id: If0e2394e5a5ead6ab165b6f45fdc59f388c0e08d
Reviewed-on: http://review.couchbase.org/25921
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Jin Lim <jin@couchbase.com>