MB-4629 Don't check CAS validity for TAP backfill items.
MB-4629 Don't check CAS validity for TAP backfill items.

As TAP backfill resets a given vbucket on the destination first
and then sends the current snapshot of that vbucket to the
destination, we should not check any CAS validness for TAP
backfill items.

Change-Id: Ie2da3ed820c54173b47fcf0bf56fe71679c75a65
Reviewed-on: http://review.couchbase.org/12611
Reviewed-by: Trond Norbye <60edd2ef23891a753f231b0c6f161dc634079a93@gmail.com>
Tested-by: Chiyoung Seo <8b279095f11c26cc21045fda5a3e51debb005aa9@gmail.com>

couchbase/ep-engine,abhinavdangeti/ep-engine,jimwwalker/ep-engine,couchbaselabs/ep-engine,sriganes/ep-engine,couchbaselabs/ep-engine,daverigby/kv_engine,hisundar/ep-engine,owendCB/ep-engine,owendCB/ep-engine,membase/ep-engine,membase/ep-engine,couchbase/ep-engine,sriganes/ep-engine,teligent-ru/ep-engine,abhinavdangeti/ep-engine,daverigby/kv_engine,abhinavdangeti/ep-engine,membase/ep-engine,couchbaselabs/ep-engine,couchbase/ep-engine,sriganes/ep-engine,daverigby/ep-engine,daverigby/kv_engine,couchbaselabs/ep-engine,hisundar/ep-engine,jimwwalker/ep-engine,daverigby/ep-engine,jimwwalker/ep-engine,teligent-ru/ep-engine,daverigby/ep-engine,teligent-ru/ep-engine,membase/ep-engine,hisundar/ep-engine,sriganes/ep-engine,hisundar/ep-engine,abhinavdangeti/ep-engine,owendCB/ep-engine,couchbase/ep-engine,daverigby/ep-engine,jimwwalker/ep-engine,abhinavdangeti/ep-engine,teligent-ru/ep-engine,owendCB/ep-engine,daverigby/kv_engine,couchbaselabs/ep-engine