Destroy incubated objects if they are not claimed when ready.

Destroy incubated objects if they are not claimed when ready.

If model data is removed while an object is incubating for it the view
cannot cancel the incubation because the index is invalidated. So if
the object has not been referenced after signalling it is ready assume
it's been abandoned and destroy it.

Change-Id: I32708e940cfbccbe0d330ad7822f29d3c1f39d13
Reviewed-by: Martin Jones <martin.jones@nokia.com>