CBD-80 delete item object before notifyIOComplete
CBD-80 delete item object before notifyIOComplete

For consistent calculation of memory use, especially on multi-core
platforms, delete dynamically allocated item object before calling
notifyIOComplete in completeBGFetch

Change-Id: I340a963eb20804dacfb5f1c61c3446ad3736312f
Reviewed-on: http://review.couchbase.org/15922
Reviewed-by: Chiyoung Seo <8b279095f11c26cc21045fda5a3e51debb005aa9@gmail.com>
Reviewed-by: Jin Lim <c45cd80a556f432a3607171a6818cd5a074b5cd4@couchbase.com>
Tested-by: Jin Lim <c45cd80a556f432a3607171a6818cd5a074b5cd4@couchbase.com>

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