MB-7026 Drop disk backfilled items filled by memory backfill
MB-7026 Drop disk backfilled items filled by memory backfill

If an item backfilled from disk is resident in memory hash
table and has the same seq num and cas value as the ones in
memory, that disk backfilled item can be simply ignored because
the same item was already (or will be) replicated by the memory
backfill operation.

Change-Id: I881872ed23bad52b1a7324a10c760be97d98a012
Reviewed-on: http://review.couchbase.org/21981
Reviewed-by: Jin Lim <c45cd80a556f432a3607171a6818cd5a074b5cd4@couchbase.com>
Tested-by: Jin Lim <c45cd80a556f432a3607171a6818cd5a074b5cd4@couchbase.com>

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