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>