Don't mutate old collection on bulk replace

Don't mutate old collection on bulk replace

For a bulk replace, assume the old collection is no longer
useful to the attribute system and only send the removal events,
not actually mutated the collection.

this changes behavior significantly and also means that dispose_collection
now receives the old collection intact.

Change-Id: Ic2685c85438191f07797d9ef97833a2cfdc4fcc2
Fixes: #3913