MB-8486 Create a new CAS for an item with corrupted CAS in warmup

If the CAS value of a given item is corrupted during warmup, we
can simply create a new CAS value for that item instead of
aborting the process.

Change-Id: I02b4889c6f71d27f98946aeeb9529f90bdbff53b
Reviewed-on: http://review.couchbase.org/27376
Reviewed-by: Jin Lim <jin@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung.seo@gmail.com>