Speed up test_access_scanner
Speed up test_access_scanner

test_access_scanner takes a long time to run
under ThreadSanitizer. The cost appears to be related to the number of
stats calls used to check the current residency ratio is.

Speed this up by only gathering stats every 100 items - this doesn't
materially affect the test, but does reduce test runtime to less than
5s under ThreadSanitizer.

Change-Id: If597a8d774bbde830a3b55a778ee9216b35e5b97
Reviewed-on: http://review.couchbase.org/55962
Tested-by: buildbot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: abhinav dangeti <d73ef2279de6577f7bf081737ee47554bbb7dc82@couchbase.com>

membase/ep-engine,owendCB/ep-engine,hisundar/ep-engine,hisundar/ep-engine,daverigby/kv_engine,daverigby/ep-engine,teligent-ru/ep-engine,couchbase/ep-engine,hisundar/ep-engine,hisundar/ep-engine,couchbase/ep-engine,membase/ep-engine,teligent-ru/ep-engine,daverigby/ep-engine,daverigby/kv_engine,couchbase/ep-engine,daverigby/ep-engine,daverigby/kv_engine,daverigby/kv_engine,daverigby/ep-engine,owendCB/ep-engine,owendCB/ep-engine,membase/ep-engine,couchbase/ep-engine,teligent-ru/ep-engine,teligent-ru/ep-engine,owendCB/ep-engine,membase/ep-engine