Refactor EventuallyPersistentStore::queueDirty().
Refactor EventuallyPersistentStore::queueDirty().

As value_t(NULL) is passed to queueDirty() for item deletion
by default, the same QueuedItem constructor can be simply
used for mutation and deletion.

Change-Id: Idf9b5a21c64af88e1062586b97b97749382547fd
Reviewed-on: http://review.couchbase.org/12153
Reviewed-by: Michael Wiederhold <a17fed27eaa842282862ff7c1b9c8395a26ac320@couchbase.com>
Tested-by: Chiyoung Seo <8b279095f11c26cc21045fda5a3e51debb005aa9@gmail.com>

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