Set the number of items per vbucket during warmup

This change reads the number of items per vbucket from the
underlying storage engine and set it per vbucket during warmup.
Especially, it is required with the full evcition as we don't
need to load all the keys and their metadata values during the
warmup.

Change-Id: Ic249db739534d73709fa618b7719aff45d4cab6a
Reviewed-on: http://review.couchbase.org/33685
Reviewed-by: abhinav dangeti <abhinav@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>
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>