A seq number should be incremented for each disk fetch operation.
A seq number should be incremented for each disk fetch operation.

Change-Id: Ib6eff966434abe9bfacbf10f068611e01db7b46d
Reviewed-on: http://review.couchbase.org/11174
Tested-by: Trond Norbye <60edd2ef23891a753f231b0c6f161dc634079a93@gmail.com>
Reviewed-by: Trond Norbye <60edd2ef23891a753f231b0c6f161dc634079a93@gmail.com>

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