MB-9306 Fix v->isDirty assertion due to race

BGFetcher must Validate CAS values before queuing dirty
item to disk when exptime is updated

Relax assertion since downside is just another set operation

Change-Id: Icfcce79371bb867052f12c8a8574c8a318d71ace
Reviewed-on: http://review.couchbase.org/29461
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>