MB-8581 Continue warmup after loading access_log.

Added a loadingData step after loadingAccessLog to meet
warmup watermarks. In order to avoid loading items already
loaded earlier in loadingAccessLog phase, we check if an
item is resident or not during loadingData phase.
A method "isLoaded" is added to LoadingStorageKVPairCallback
to perform the check.

Added a test case "warmup_accesslog" for this feature.

Change-Id: I5eeb86b43b4e05a87df97b103c253a2db79db5ed
Reviewed-on: http://review.couchbase.org/28038
Reviewed-by: Chiyoung Seo <chiyoung.seo@gmail.com>
Tested-by: Chiyoung Seo <chiyoung.seo@gmail.com>