Add of new prepare replaces unpersisted delete which exposes old document to bgfetch in FullEviction

In the VBucket::add code we allow an unpersisted delete to be replaced. If we replace an unpersisted delete with a committed item then all works fine as that item can be returned from the HashTable. If we replace an unpersisted delete with a prepare then a get finds no item in the HashTable and may queue a BgFetch which can return the document in its previous (undeleted) state.