Stop the access log loading task if the warmup threshold reached

If any of warmup thresholds (i.e., memory, num of items loaded),
reached while loading items through the access log, then the
warmup task should be terminated immediately.

Change-Id: If34f06fa86303592378737723a7eef09514c43b0
Reviewed-on: http://review.couchbase.org/33686
Reviewed-by: abhinav dangeti <abhinav@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>