MB-11876 Safely cancel AccessScanner task on shutdown

Change-Id: I769ca2ad562ef2b27447d40b6a66c4e24ff21eb3
Reviewed-on: http://review.couchbase.org/40669
Tested-by: Sundararaman Sridharan <sundar@couchbase.com>
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>