MB-11847: [Warmup] Estimated value count with full eviction

In loadingKVPairs phase, set estimated value count equal
to the estimated item count, as no keys have been warmed
up yet, when in full eviction mode.

Change-Id: Id74237c96e0c144d9a9b94e0670f4975739e7914
Reviewed-on: http://review.couchbase.org/40304
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: abhinav dangeti <abhinav@couchbase.com>