MB-12100: Don't expire items for replica vbuckets during warmup

Change-Id: I81095beb69c0996d819158e3cadc451a83653d2f
Reviewed-on: http://review.couchbase.org/41177
Reviewed-by: Sundararaman Sridharan <sundar@couchbase.com>
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>